lpwan Working Group A. Minaburo
Internet-Draft Acklio
Intended status: Standards Track L. Toutain
Expires: April 25, 2019 IMT-Atlantique
C. Gomez
Universitat Politecnica de Catalunya
D. Barthel
Orange Labs
JC. Zuniga
SIGFOX
October 22, 2018
LPWAN Static Context Header Compression (SCHC) and fragmentation for
IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-17
Abstract
This document defines the Static Context Header Compression (SCHC)
framework, which provides both header compression and fragmentation
functionalities. SCHC has been designed for Low Power Wide Area
Networks (LPWAN).
SCHC compression is based on a common static context stored in both
the LPWAN device and the network side. This document defines a
header compression mechanism and its application to compress IPv6/UDP
headers.
This document also specifies a fragmentation and reassembly mechanism
that is used to support the IPv6 MTU requirement over the LPWAN
technologies. Fragmentation is needed for IPv6 datagrams that, after
SCHC compression or when such compression was not possible, still
exceed the layer-2 maximum payload size.
The SCHC header compression and fragmentation mechanisms are
independent of the specific LPWAN technology over which they are
used. This document defines generic functionalities and offers
flexibility with regard to parameter settings and mechanism choices.
Technology-specific and product-specific settings and choices are
expected to be grouped into Profiles specified in other documents.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. SCHC overview . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. SCHC Packet format . . . . . . . . . . . . . . . . . . . 10
5.2. Functional mapping . . . . . . . . . . . . . . . . . . . 11
6. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Compression/Decompression . . . . . . . . . . . . . . . . . . 12
7.1. SCHC C/D Rules . . . . . . . . . . . . . . . . . . . . . 12
7.2. Rule ID for SCHC C/D . . . . . . . . . . . . . . . . . . 14
7.3. Packet processing . . . . . . . . . . . . . . . . . . . . 15
7.4. Matching operators . . . . . . . . . . . . . . . . . . . 16
7.5. Compression Decompression Actions (CDA) . . . . . . . . . 17
7.5.1. processing variable-length fields . . . . . . . . . . 17
7.5.2. not-sent CDA . . . . . . . . . . . . . . . . . . . . 18
7.5.3. value-sent CDA . . . . . . . . . . . . . . . . . . . 18
7.5.4. mapping-sent CDA . . . . . . . . . . . . . . . . . . 18
7.5.5. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19
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7.5.6. DevIID, AppIID CDA . . . . . . . . . . . . . . . . . 19
7.5.7. Compute-* . . . . . . . . . . . . . . . . . . . . . . 19
8. Fragmentation/Reassembly . . . . . . . . . . . . . . . . . . 20
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20
8.2. SCHC F/R Tools . . . . . . . . . . . . . . . . . . . . . 20
8.2.1. Messages . . . . . . . . . . . . . . . . . . . . . . 20
8.2.2. Tiles, Windows, Bitmaps, Timers, Counters . . . . . . 21
8.2.3. Integrity Checking . . . . . . . . . . . . . . . . . 23
8.2.4. Header Fields . . . . . . . . . . . . . . . . . . . . 24
8.3. SCHC F/R Message Formats . . . . . . . . . . . . . . . . 26
8.3.1. SCHC Fragment format . . . . . . . . . . . . . . . . 26
8.3.2. SCHC ACK format . . . . . . . . . . . . . . . . . . . 27
8.3.3. SCHC ACK REQ format . . . . . . . . . . . . . . . . . 30
8.3.4. SCHC Abort formats . . . . . . . . . . . . . . . . . 31
8.4. SCHC F/R modes . . . . . . . . . . . . . . . . . . . . . 33
8.4.1. No-ACK mode . . . . . . . . . . . . . . . . . . . . . 33
8.4.2. ACK-Always . . . . . . . . . . . . . . . . . . . . . 36
8.4.3. ACK-on-Error . . . . . . . . . . . . . . . . . . . . 42
9. Padding management . . . . . . . . . . . . . . . . . . . . . 49
10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 50
10.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 50
10.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 51
10.3. Flow label field . . . . . . . . . . . . . . . . . . . . 51
10.4. Payload Length field . . . . . . . . . . . . . . . . . . 51
10.5. Next Header field . . . . . . . . . . . . . . . . . . . 52
10.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . 52
10.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . 52
10.7.1. IPv6 source and destination prefixes . . . . . . . . 52
10.7.2. IPv6 source and destination IID . . . . . . . . . . 53
10.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . 53
10.9. UDP source and destination port . . . . . . . . . . . . 53
10.10. UDP length field . . . . . . . . . . . . . . . . . . . . 54
10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 54
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55
12. Security considerations . . . . . . . . . . . . . . . . . . . 55
12.1. Security considerations for SCHC
Compression/Decompression . . . . . . . . . . . . . . . 55
12.2. Security considerations for SCHC
Fragmentation/Reassembly . . . . . . . . . . . . . . . . 55
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 57
14.1. Normative References . . . . . . . . . . . . . . . . . . 57
14.2. Informative References . . . . . . . . . . . . . . . . . 57
Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 58
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 61
Appendix C. Fragmentation State Machines . . . . . . . . . . . . 68
Appendix D. SCHC Parameters . . . . . . . . . . . . . . . . . . 75
Appendix E. Supporting multiple window sizes for fragmentation . 77
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Appendix F. Downlink SCHC Fragment transmission . . . . . . . . 77
Appendix G. Note . . . . . . . . . . . . . . . . . . . . . . . . 78
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 78
1. Introduction
This document defines the Static Context Header Compression (SCHC)
framework, which provides both header compression and fragmentation
functionalities. SCHC has been designed for Low Power Wide Area
Networks (LPWAN).
Header compression is needed for efficient Internet connectivity to
the node within an LPWAN network. Some LPWAN networks properties can
be exploited to get an efficient header compression:
o The network topology is star-oriented, which means that all
packets between the same source-destination pair follow the same
path. For the needs of this document, the architecture can simply
be described as Devices (Dev) exchanging information with LPWAN
Application Servers (App) through a Network Gateway (NGW).
o Because devices embed built-in applications, the traffic flows to
be compressed are known in advance. Indeed, new applications are
less frequently installed in an LPWAN device, as they are in a
computer or smartphone.
SCHC compression uses a context in which information about header
fields is stored. This context is static: the values of the header
fields do not change over time. This avoids complex
resynchronization mechanisms. Indeed, downlink is often more
restricted/expensive, perhaps completely unavailable [RFC8376]. A
compression protocol that relies on feedback is not compatible with
the characteristics of such LPWANs.
In most cases, a small context identifier is enough to represent the
full IPv6/UDP headers. The SCHC header compression mechanism is
independent of the specific LPWAN technology over which it is used.
LPWAN technologies impose some strict limitations on traffic. For
instance, devices are sleeping most of the time and may receive data
during short periods of time after transmission to preserve battery.
LPWAN technologies are also characterized by a greatly reduced data
unit and/or payload size (see [RFC8376]). However, some LPWAN
technologies do not provide fragmentation functionality; to support
the IPv6 MTU requirement of 1280 bytes [RFC8200], they require a
fragmentation protocol at the adaptation layer below IPv6.
Accordingly, this document defines an fragmentation/reassembly
mechanism for LPWAN technologies to supports the IPv6 MTU. Its
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implementation is optional. If not interested, the reader can safely
skip its description.
This document defines generic functionality and offers flexibility
with regard to parameters settings and mechanism choices.
Technology-specific settings and product-specific and choices are
expected to be grouped into Profiles specified in other documents.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. LPWAN Architecture
LPWAN technologies have similar network architectures but different
terminologies. Using the terminology defined in [RFC8376], we can
identify different types of entities in a typical LPWAN network, see
Figure 1:
o Devices (Dev) are the end-devices or hosts (e.g. sensors,
actuators, etc.). There can be a very high density of devices per
radio gateway.
o The Radio Gateway (RGW), which is the end point of the constrained
link.
o The Network Gateway (NGW) is the interconnection node between the
Radio Gateway and the Internet.
o LPWAN-AAA Server, which controls the user authentication and the
applications.
o Application Server (App)
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+------+
() () () | |LPWAN-|
() () () () / \ +---------+ | AAA |
() () () () () () / \======| ^ |===|Server| +-----------+
() () () | | | +------+ |APPLICATION|
() () () () / \==========| v |=============| (App) |
() () () / \ +---------+ +-----------+
Dev Radio Gateways NGW
Figure 1: LPWAN Architecture
4. Terminology
This section defines the terminology and acronyms used in this
document.
Note that the SCHC acronym is pronounced like "sheek" in English (or
"chic" in French). Therefore, this document writes "a SCHC Packet"
instead of "an SCHC Packet".
o App: LPWAN Application. An application sending/receiving IPv6
packets to/from the Device.
o AppIID: Application Interface Identifier. The IID that identifies
the application server interface.
o Bi: Bidirectional. Characterises a Field Descriptor that applies
to headers of packets travelling in either direction (Up and Dw,
see this glossary).
o CDA: Compression/Decompression Action. Describes the reciprocal
pair of actions that are performed at the compressor to compress a
header field and at the decompressor to recover the original
header field value.
o Compression Residue. The bits that need to be sent (beyond the
Rule ID itself) after applying the SCHC compression over each
header field.
o Context: A set of Rules used to compress/decompress headers.
o Dev: Device. A node connected to an LPWAN. A Dev SHOULD
implement SCHC.
o DevIID: Device Interface Identifier. The IID that identifies the
Dev interface.
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o DI: Direction Indicator. This field tells which direction of
packet travel (Up, Dw or Bi) a Rule applies to. This allows for
assymmetric processing.
o Dw: Downlink direction for compression/decompression in both
sides, from SCHC C/D in the network to SCHC C/D in the Dev.
o Field Description. A line in the Rule table.
o FID: Field Identifier. This is an index to describe the header
fields in a Rule.
o FL: Field Length is the length of the packet header field. It is
expressed in bits for header fields of fixed lengths or as a type
(e.g. variable, token length, ...) for field lengths that are
unknown at the time of Rule creation. The length of a header
field is defined in the corresponding protocol specification (such
as IPv6 or UDP).
o FP: Field Position is a value that is used to identify the
position where each instance of a field appears in the header.
o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136]
o L2: Layer two. The immediate lower layer SCHC interfaces with.
It is provided by an underlying LPWAN technology. It does not
necessarily correspond to the OSI model definition of Layer 2.
o L2 Word: this is the minimum subdivision of payload data that the
L2 will carry. In most L2 technologies, the L2 Word is an octet.
In bit-oriented radio technologies, the L2 Word might be a single
bit. The L2 Word size is assumed to be constant over time for
each device.
o MO: Matching Operator. An operator used to match a value
contained in a header field with a value contained in a Rule.
o Padding (P). Extra bits that may be appended by SCHC to a data
unit that it passes to the underlying Layer 2 for transmission.
SCHC itself operates on bits, not bytes, and does not have any
alignment prerequisite. See Section 9.
o Profile: SCHC offers variations in the way it is operated, with a
number of parameters listed in Appendix D. A Profile indicates a
particular setting of all these parameters. Both ends of a SCHC
session must be provisioned with the same Profile information and
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with the same set of Rules before the session starts, so that
there is no ambiguity in how they expect to communicate.
o Rule: A set of header field values.
o Rule ID: An identifier for a Rule. SCHC C/D on both sides share
the same Rule ID for a given packet. A set of Rule IDs are used
to support SCHC F/R functionality.
o SCHC C/D: Static Context Header Compression Compressor/
Decompressor. A mechanism used on both sides, at the Dev and at
the network, to achieve Compression/Decompression of headers.
SCHC C/D uses Rules to perform compression and decompression.
o SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been
compressed as per the header compression mechanism defined in this
document. If the header compression process is unable to actually
compress the packet header, the packet with the uncompressed
header is still called a SCHC Packet (in this case, a Rule ID is
used to indicate that the packet header has not been compressed).
See Section 7 for more details.
o TV: Target value. A value contained in a Rule that will be
matched with the value of a header field.
o Up: Uplink direction for compression/decompression in both sides,
from the Dev SCHC C/D to the network SCHC C/D.
Additional terminology for the optional SCHC Fragmentation /
Reassembly mechanism (SCHC F/R) is found in Section 8.2.
5. SCHC overview
SCHC can be characterized as an adaptation layer between IPv6 and the
underlying LPWAN technology. SCHC comprises two sublayers (i.e. the
Compression sublayer and the Fragmentation sublayer), as shown in
Figure 2.
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+----------------+
| IPv6 |
+- +----------------+
| | Compression |
SCHC < +----------------+
| | Fragmentation |
+- +----------------+
|LPWAN technology|
+----------------+
Figure 2: Protocol stack comprising IPv6, SCHC and an LPWAN
technology
As per this document, when a packet (e.g. an IPv6 packet) needs to be
transmitted, header compression is first applied to the packet. The
resulting packet after header compression (whose header may or may
not actually be smaller than that of the original packet) is called a
SCHC Packet. If the SCHC Packet needs to be fragmented by the
optional SCHC Fragmentation, fragmentation is then applied to the
SCHC Packet. The SCHC Packet or the SCHC Fragments are then
transmitted over the LPWAN. The reciprocal operations take place at
the receiver. This process is illustrated in Figure 3.
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A packet (e.g. an IPv6 packet)
| ^
v |
+------------------+ +--------------------+
| SCHC Compression | | SCHC Decompression |
+------------------+ +--------------------+
| ^
| If no fragmentation (*) |
+-------------- SCHC Packet -------------->|
| |
v |
+--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+
| ^ | ^
| | | |
| +-------------- SCHC ACK -------------+ |
| |
+-------------- SCHC Fragments -------------------+
SENDER RECEIVER
*: the decision to use Fragmentation or not is left to each Profile.
Figure 3: SCHC operations at the SENDER and the RECEIVER
5.1. SCHC Packet format
The SCHC Packet is composed of the Compressed Header followed by the
payload from the original packet (see Figure 4). The Compressed
Header itself is composed of the Rule ID and a Compression Residue,
which is the output of the compression actions of the Rule that was
applied (see Section 7). The Compression Residue may be empty. Both
the Rule ID and the Compression Residue potentially have a variable
size, and generally are not a mutiple of bytes in size.
| Rule ID + Compression Residue |
+---------------------------------+--------------------+
| Compressed Header | Payload |
+---------------------------------+--------------------+
Figure 4: SCHC Packet
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5.2. Functional mapping
Figure 5 below maps the functional elements of Figure 3 onto the
LPWAN architecture elements of Figure 1.
Dev App
+----------------+ +--------------+
| APP1 APP2 APP3 | |APP1 APP2 APP3|
| | | |
| UDP | | UDP |
| IPv6 | | IPv6 |
| | | |
|SCHC C/D and F/R| | |
+--------+-------+ +-------+------+
| +--+ +----+ +-----------+ .
+~~ |RG| === |NGW | === | SCHC |... Internet ..
+--+ +----+ |F/R and C/D|
+-----------+
Figure 5: Architecture
SCHC C/D and SCHC F/R are located on both sides of the LPWAN
transmission, i.e. on the Dev side and on the Network side.
The operation in the Uplink direction is as follows. The Device
application uses IPv6 or IPv6/UDP protocols. Before sending the
packets, the Dev compresses their headers using SCHC C/D and, if the
SCHC Packet resulting from the compression needs to be fragmented by
SCHC, SCHC F/R is performed (see Section 8). The resulting SCHC
Fragments are sent to an LPWAN Radio Gateway (RG) which forwards them
to a Network Gateway (NGW). The NGW sends the data to a SCHC F/R for
re-assembly (if needed) and then to the SCHC C/D for decompression.
After decompression, the packet can be sent over the Internet to one
or several LPWAN Application Servers (App).
The SCHC F/R and C/D on the Network side can be located in the NGW,
or somewhere else as long as a tunnel is established between them and
the NGW. Note that, for some LPWAN technologies, it MAY be suitable
to locate the SCHC F/R functionality nearer the NGW, in order to
better deal with time constraints of such technologies.
The SCHC C/D and F/R on both sides MUST share the same set of Rules.
The SCHC C/D and F/R process is symmetrical, therefore the
description of the Downlink direction is symmetrical to the one
above.
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6. Rule ID
Rule IDs are identifiers used to select the correct context either
for Compression/Decompression or for Fragmentation/Reassembly.
The size of the Rule IDs is not specified in this document, as it is
implementation-specific and can vary according to the LPWAN
technology and the number of Rules, among others. It is defined in
Profiles.
The Rule IDs are used:
o In the SCHC C/D context, to identify the Rule (i.e., the set of
Field Descriptions) that is used to compress a packet header.
o At least one Rule ID MAY be allocated to tagging packets for which
SCHC compression was not possible (no matching Rule was found).
o In SCHC F/R, to identify the specific modes and settings of SCHC
Fragments being transmitted, and to identify the SCHC ACKs,
including their modes and settings. Note that when F/R is used
for both communication directions, at least two Rule ID values are
therefore needed for F/R.
7. Compression/Decompression
Compression with SCHC is based on using context, i.e. a set of Rules
to compress or decompress headers. SCHC avoids context
synchronization, which consumes considerable bandwidth in other
header compression mechanisms such as RoHC [RFC5795]. Since the
content of packets is highly predictable in LPWAN networks, static
contexts MAY be stored beforehand to omit transmitting some
information over the air. The contexts MUST be stored at both ends,
and they can be learned by a provisioning protocol or by out of band
means, or they can be pre-provisioned. The way the contexts are
provisioned is out of the scope of this document.
7.1. SCHC C/D Rules
The main idea of the SCHC compression scheme is to transmit the Rule
ID to the other end instead of sending known field values. This Rule
ID identifies a Rule that provides the closest match to the original
packet values. Hence, when a value is known by both ends, it is only
necessary to send the corresponding Rule ID over the LPWAN network.
The manner by which Rules are generated is out of the scope of this
document. The Rules MAY be changed at run-time but the mechanism is
out of scope of this document.
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The context contains a list of Rules (see Figure 6). Each Rule
itself contains a list of Field Descriptions composed of a Field
Identifier (FID), a Field Length (FL), a Field Position (FP), a
Direction Indicator (DI), a Target Value (TV), a Matching Operator
(MO) and a Compression/Decompression Action (CDA).
/-----------------------------------------------------------------\
| Rule N |
/-----------------------------------------------------------------\|
| Rule i ||
/-----------------------------------------------------------------\||
| (FID) Rule 1 |||
|+-------+--+--+--+------------+-----------------+---------------+|||
||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+--+------------+-----------------+---------------+|||
||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+--+------------+-----------------+---------------+|||
||... |..|..|..| ... | ... | ... ||||
|+-------+--+--+--+------------+-----------------+---------------+||/
||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
|+-------+--+--+--+------------+-----------------+---------------+|/
| |
\-----------------------------------------------------------------/
Figure 6: A Compression/Decompression Context
A Rule does not describe how to parse a packet header to find each
field. This MUST be known from the compressor/decompressor. Rules
only describe the compression/decompression behavior for each header
field. In a Rule, the Field Descriptions are listed in the order in
which the fields appear in the packet header.
A Rule also describes what is sent in the Compression Residue. The
Compression Residue is assembled by concatenating the residues for
each field, in the order the Field Descriptions appear in the Rule.
The Context describes the header fields and its values with the
following entries:
o Field ID (FID) is a unique value to define the header field.
o Field Length (FL) represents the length of the field. It can be
either a fixed value (in bits) if the length is known when the
Rule is created or a type if the length is variable. The length
of a header field is defined in the corresponding protocol
specification. The type defines the process to compute the
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length, its unit (bits, bytes,...) and the value to be sent before
the Compression Residue.
o Field Position (FP): most often, a field only occurs once in a
packet header. Some fields may occur multiple times in a header.
FP indicates which occurrence this Field Description applies to.
The default value is 1 (first occurence).
o A Direction Indicator (DI) indicates the packet direction(s) this
Field Description applies to. Three values are possible:
* UPLINK (Up): this Field Description is only applicable to
packets sent by the Dev to the App,
* DOWNLINK (Dw): this Field Description is only applicable to
packets sent from the App to the Dev,
* BIDIRECTIONAL (Bi): this Field Description is applicable to
packets travelling both Up and Dw.
o Target Value (TV) is the value used to match against the packet
header field. The Target Value can be of any type (integer,
strings, etc.). It can be a single value or a more complex
structure (array, list, etc.), such as a JSON or a CBOR structure.
o Matching Operator (MO) is the operator used to match the Field
Value and the Target Value. The Matching Operator may require
some parameters. MO is only used during the compression phase.
The set of MOs defined in this document can be found in
Section 7.4.
o Compression Decompression Action (CDA) describes the compression
and decompression processes to be performed after the MO is
applied. Some CDAs MAY require parameter values for their
operation. CDAs are used in both the compression and the
decompression functions. The set of CDAs defined in this document
can be found in Section 7.5.
7.2. Rule ID for SCHC C/D
Rule IDs are sent by the compression function in one side and are
received for the decompression function in the other side. In SCHC
C/D, the Rule IDs are specific to a Dev. Hence, multiple Dev
instances MAY use the same Rule ID to define different header
compression contexts. To identify the correct Rule ID, the SCHC C/D
needs to associate the Rule ID with the Dev identifier to find the
appropriate Rule to be applied.
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7.3. Packet processing
The compression/decompression process follows several steps:
o Compression Rule selection: The goal is to identify which Rule(s)
will be used to compress the packet's headers. When performing
decompression, on the network side the SCHC C/D needs to find the
correct Rule based on the L2 address; in this way, it can use the
DevIID and the Rule ID. On the Dev side, only the Rule ID is
needed to identify the correct Rule since the Dev typically only
holds Rules that apply to itself. The Rule will be selected by
matching the Fields Descriptions to the packet header as described
below. When the selection of a Rule is done, this Rule is used to
compress the header. The detailed steps for compression Rule
selection are the following:
* The first step is to choose the Field Descriptions by their
direction, using the Direction Indicator (DI). A Field
Description that does not correspond to the appropriate DI will
be ignored. If all the fields of the packet do not have a
Field Description with the correct DI, the Rule is discarded
and SCHC C/D proceeds to consider the next Rule.
* When the DI has matched, then the next step is to identify the
fields according to Field Position (FP). If FP does not
correspond, the Rule is not used and the SCHC C/D proceeds to
consider the next Rule.
* Once the DI and the FP correspond to the header information,
each packet field's value is then compared to the corresponding
Target Value (TV) stored in the Rule for that specific field
using the matching operator (MO).
If all the fields in the packet's header satisfy all the
matching operators (MO) of a Rule (i.e. all MO results are
True), the fields of the header are then compressed according
to the Compression/Decompression Actions (CDAs) and a
compressed header (with possibly a Compression Residue) SHOULD
be obtained. Otherwise, the next Rule is tested.
* If no eligible compression Rule is found, then the header MUST
be sent without compression, using a Rule ID dedicated to this
purpose. Sending the header uncompressed but may require the
use of the SCHC F/R process.
o Sending: The Rule ID is sent to the other end followed by the
Compression Residue (which could be empty) or the uncompressed
header, and directly followed by the payload. The Compression
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Residue is the concatenation of the Compression Residues for each
field according to the CDAs for that Rule. The way the Rule ID is
sent depends on the Profile. For example, it can be either
included in an L2 header or sent in the first byte of the L2
payload. (see Figure 4). This process will be specified in the
Profile and is out of the scope of the present document. On LPWAN
technologies that are byte-oriented, the compressed header
concatenated with the original packet payload is padded to a
multiple of 8 bits, if needed. See Section 9 for details.
o Decompression: When doing decompression, on the network side the
SCHC C/D needs to find the correct Rule based on the L2 address
and in this way, it can use the DevIID and the Rule ID. On the
Dev side, only the Rule ID is needed to identify the correct Rule
since the Dev only holds Rules that apply to itself.
The receiver identifies the sender through its device-id or source
identifier (e.g. MAC address, if it exists) and selects the Rule
using the Rule ID. This Rule describes the compressed header
format and associates the received Compression Residue to each of
the header fields. For each field in the header, the receiver
applies the CDA action associated to that field in order to
reconstruct the original header field value. The CDA application
order can be different from the order in which the fields are
listed in the Rule. In particular, Compute-* MUST be applied
after the application of the CDAs of all the fields it computes
on.
7.4. Matching operators
Matching Operators (MOs) are functions used by both SCHC C/D
endpoints involved in the header compression/decompression. They are
not typed and can be applied to integer, string or any other data
type. The result of the operation can either be True or False. MOs
are defined as follows:
o equal: The match result is True if the field value in the packet
matches the TV.
o ignore: No check is done between the field value in the packet and
the TV in the Rule. The result of the matching is always true.
o MSB(x): A match is obtained if the most significant x bits of the
packet header field value are equal to the TV in the Rule. The x
parameter of the MSB MO indicates how many bits are involved in
the comparison. If the FL is described as variable, the length
must be a multiple of the unit. For example, x must be multiple
of 8 if the unit of the variable length is in bytes.
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o match-mapping: With match-mapping, the Target Value is a list of
values. Each value of the list is identified by a short ID (or
index). Compression is achieved by sending the index instead of
the original header field value. This operator matches if the
header field value is equal to one of the values in the target
list.
7.5. Compression Decompression Actions (CDA)
The Compression Decompression Action (CDA) describes the actions
taken during the compression of headers fields, and inversely, the
action taken by the decompressor to restore the original value.
/--------------------+-------------+----------------------------\
| Action | Compression | Decompression |
| | | |
+--------------------+-------------+----------------------------+
|not-sent |elided |use value stored in context |
|value-sent |send |build from received value |
|mapping-sent |send index |value from index on a table |
|LSB |send LSB |TV, received value |
|compute-length |elided |compute length |
|compute-checksum |elided |compute UDP checksum |
|DevIID |elided |build IID from L2 Dev addr |
|AppIID |elided |build IID from L2 App addr |
\--------------------+-------------+----------------------------/
Figure 7: Compression and Decompression Actions
Figure 7 summarizes the basic actions that can be used to compress
and decompress a field. The first column shows the action's name.
The second and third columns show the reciprocal compression/
decompression behavior for each action.
Compression is done in the order that the Field Descriptions appear
in a Rule. The result of each Compression/Decompression Action is
appended to the accumulated Compression Residue in that same order.
The receiver knows the size of each compressed field, which can be
given by the Rule or MAY be sent with the compressed header.
7.5.1. processing variable-length fields
If the field is identified in the Field Description as being of
variable size, then the size of the Compression Residue value (using
the unit defined in the FL) MUST first be sent as follows:
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o If the size is between 0 and 14, it is sent as a 4-bits unsigned
integer.
o For values between 15 and 254, 0b1111 is transmitted and then the
size is sent as an 8 bits unsigned integer.
o For larger values of the size, 0xfff is transmitted and then the
next two bytes contain the size value as a 16 bits unsigned
integer.
If a field is not present in the packet but exists in the Rule and
its FL is specified as being variable, size 0 MUST be sent to denote
its absence.
7.5.2. not-sent CDA
The not-sent action is generally used when the field value is
specified in a Rule and therefore known by both the Compressor and
the Decompressor. This action SHOULD be used with the "equal" MO.
If MO is "ignore", there is a risk to have a decompressed field value
different from the original field that was compressed.
The compressor does not send any Compression Residue for a field on
which not-sent compression is applied.
The decompressor restores the field value with the Target Value
stored in the matched Rule identified by the received Rule ID.
7.5.3. value-sent CDA
The value-sent action is generally used when the field value is not
known by both the Compressor and the Decompressor. The value is sent
as a residue in the compressed message header. Both Compressor and
Decompressor MUST know the size of the field, either implicitly (the
size is known by both sides) or by explicitly indicating the length
in the Compression Residue, as defined in Section 7.5.1. This action
is generally used with the "ignore" MO.
7.5.4. mapping-sent CDA
The mapping-sent action is used to send an index (the index into the
Target Value list of values) instead of the original value. This
action is used together with the "match-mapping" MO.
On the compressor side, the match-mapping Matching Operator searches
the TV for a match with the header field value and the mapping-sent
CDA appends the corresponding index to the Compression Residue to be
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sent. On the decompressor side, the CDA uses the received index to
restore the field value by looking up the list in the TV.
The number of bits sent is the minimal size for coding all the
possible indices.
7.5.5. LSB CDA
The LSB action is used together with the "MSB(x)" MO to avoid sending
the most significant part of the packet field if that part is already
known by the receiving end. The number of bits sent is the original
header field length minus the length specified in the MSB(x) MO.
The compressor sends the Least Significant Bits (e.g. LSB of the
length field). The decompressor concatenates the x most significant
bits of Target Value and the received residue.
If this action needs to be done on a variable length field, the size
of the Compression Residue in bytes MUST be sent as described in
Section 7.5.1.
7.5.6. DevIID, AppIID CDA
These actions are used to process respectively the Dev and the App
Interface Identifiers (DevIID and AppIID) of the IPv6 addresses.
AppIID CDA is less common since most current LPWAN technologies
frames contain a single L2 address, which is the Dev's address.
The IID value MAY be computed from the Device ID present in the L2
header, or from some other stable identifier. The computation is
specific to each Profile and MAY depend on the Device ID size.
In the downlink direction (Dw), at the compressor, the DevIID CDA may
be used to generate the L2 addresses on the LPWAN, based on the
packet's Destination Address.
7.5.7. Compute-*
Some fields may be elided during compression and reconstructed during
decompression. This is the case for length and checksum, so:
o compute-length: computes the length assigned to this field. This
CDA MAY be used to compute IPv6 length or UDP length.
o compute-checksum: computes a checksum from the information already
received by the SCHC C/D. This field MAY be used to compute UDP
checksum.
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8. Fragmentation/Reassembly
8.1. Overview
In LPWAN technologies, the L2 MTU typically ranges from tens to
hundreds of bytes. Some of these technologies do not have an
internal fragmentation/reassembly mechanism.
The SCHC Fragmentation/Reassembly (SCHC F/R) functionality is offered
as an option for such LPWAN technologies to cope with the IPv6 MTU
requirement of 1280 bytes [RFC8200]. It is optional to implement.
If it is not needed, its description can be safely ignored.
This specification includes several SCHC F/R modes, which allow for a
range of reliability options such as optional SCHC Fragment
retransmission. More modes may be defined in the future.
The same SCHC F/R mode MUST be used for all SCHC Fragments of the
same fragmented SCHC Packet. This document does not make any
decision with regard to which mode(s) will be used over a specific
LPWAN technology. This will be defined in Profiles.
SCHC F/R uses the knowledge of the L2 Word size (see Section 4) to
encode some messages. Therefore, SCHC MUST know the L2 Word size.
SCHC F/R usually generates SCHC Fragments and SCHC ACKs that are
multiples of L2 Words. The padding overhead is kept to the absolute
minimum (see Section 9).
8.2. SCHC F/R Tools
This subsection describes the different tools that are used to enable
the SCHC F/R functionality defined in this document. These tools
include the SCHC F/R messages, tiles, windows, counters, timers and
header fields.
The tools are described here in a generic manner. Their application
to each SCHC F/R mode is found in Section 8.4.
8.2.1. Messages
The messages that can be used by SCHC F/R are the following:
o SCHC Fragment: A data unit that carries a piece of a SCHC Packet
from the sender to the receiver.
o SCHC ACK: An acknowledgement for fragmentation, by the receiver to
the sender. This message is used to report on the successful
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reception of pieces of, or the whole of the fragmented SCHC
Packet.
o SCHC ACK REQ: An explicit request for a SCHC ACK. By the sender
to the receiver.
o SCHC Sender-Abort: A message by the sender telling the receiver
that it has aborted the transmission of a fragmented SCHC Packet.
o SCHC Receiver-Abort: A message by the receiver to tell the sender
to abort the transmission of a fragmented SCHC Packet.
8.2.2. Tiles, Windows, Bitmaps, Timers, Counters
8.2.2.1. Tiles
The SCHC Packet is fragmented into pieces, hereafter called tiles.
The tiles MUST be contiguous.
See Figure 8 for an example.
SCHC Packet
+----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Tiles | | | | | | | | | | | | | |
+----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Figure 8: a SCHC Packet fragmented in tiles
Each SCHC Fragment message carries at least one tile in its Payload,
if the Payload field is present.
8.2.2.2. Windows
Some SCHC F/R modes may handle successive tiles in groups, called
windows.
If windows are used
o all the windows of a SCHC Packet, except the last one, MUST
contain the same number of tiles. This number is WINDOW_SIZE.
o WINDOW_SIZE MUST be specified in a Profile.
o the windows are numbered.
o their numbers MUST increase from 0 upward, from the start of the
SCHC Packet to its end.
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o the last window MUST contain WINDOW_SIZE tiles or less.
o tiles are numbered within each window.
o the tile numbers MUST decrement from WINDOW_SIZE - 1 downward,
looking from the start of the SCHC Packet toward its end.
o each tile of a SCHC Packet is therefore uniquely identified by a
window number and a tile number within this window.
See Figure 9 for an example.
+---------------------------------------------...-------------+
| SCHC Packet |
+---------------------------------------------...-------------+
Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 | 3 |
Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|-- 28 -|
Figure 9: a SCHC Packet fragmented in tiles grouped in 28 windows,
with WINDOW_SIZE = 5
When windows are used
o information on the correct reception of the tiles belonging to the
same window MUST be grouped together.
o it is RECOMMENDED that this information is kept in Bitmaps.
o Bitmaps MAY be sent back to the sender in a SCHC ACK message.
o Each window has a Bitmap.
8.2.2.3. Bitmaps
Each bit in the Bitmap for a window corresponds to a tile in the
window. Each Bitmap has therefore WINDOW_SIZE bits. The bit at the
left-most position corresponds to the tile numbered WINDOW_SIZE - 1.
Consecutive bits, going right, correspond to sequentially decreasing
tile numbers. In Bitmaps for windows that are not the last one of a
SCHC Packet, the bit at the right-most position corresponds to the
tile numbered 0. In the Bitmap for the last window, the bit at the
right-most position corresponds either to the tile numbered 0 or to a
tile that is sent/received as "the last one of the SCHC Packet"
without explicitely stating its number (see Section 8.3.1.2).
At the receiver
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o a bit set to 1 in the Bitmap indicates that a tile associated with
that bit position has been correctly received for that window.
o a bit set to 0 in the Bitmap indicates that no tile associated
with that bit position has been correctly received for that
window.
WINDOW_SIZE finely controls the size of the Bitmap sent in the SCHC
ACK message, which may be critical to some LPWAN technologies.
8.2.2.4. Timers and counters
Some SCHC F/R modes can use the following timers and counters
o Inactivity Timer: this timer can be used to unlock a SCHC Fragment
receiver that is not receiving a SCHC F/R message while it is
expecting one.
o Retransmission Timer: this timer can be used by a SCHC Fragment
sender to set a timeout on expecting a SCHC ACK.
o Attempts: this counter counts the requests for SCHC ACKs.
MAX_ACK_REQUESTS is the threshold at which an exception is raised.
8.2.3. Integrity Checking
The reassembled SCHC Packet is checked for integrity at the receive
end. Integrity checking is performed by computing a MIC at the
sender side and transmitting it to the receiver for comparison with
the locally computed MIC.
The MIC supports UDP checksum elision by SCHC C/D (see
Section 10.11).
The CRC32 polynomial 0xEDB88320 (i.e. the reverse representation of
the polynomial used e.g. in the Ethernet standard [RFC3385]) is
RECOMMENDED as the default algorithm for computing the MIC.
Nevertheless, other MIC lengths or other algorithms MAY be required
by the Profile.
Note that the concatenation of the complete SCHC Packet and the
potential padding bits of the last SCHC Fragment does not generally
constitute an integer number of bytes. For implementers to be able
to use byte-oriented CRC libraries, it is RECOMMENDED that the
concatenation of the complete SCHC Packet and the last fragment
potential padding bits be zero-extended to the next byte boundary and
that the MIC be computed on that byte array. A Profile MAY specify
another behaviour.
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8.2.4. Header Fields
The SCHC F/R messages use the following fields (see the related
formats in Section 8.3):
o Rule ID: this field is present in all the SCHC F/R messages. It
is used to identify
* that a SCHC F/R message is being carried, as opposed to an
unfragmented SCHC Packet,
* which SCHC F/R mode is used
* and among this mode
+ if windows are used and what the value of WINDOW_SIZE is,
+ what other optional fields are present and what the field
sizes are.
Therefore, the Rule ID allows SCHC F/R interleaving non-fragmented
SCHC Packets and SCHC Fragments that carry other SCHC Packets, or
interleaving SCHC Fragments that use different SCHC F/R modes or
different parameters.
o Datagram Tag (DTag). The DTag field is optional. Its presence
and size (called T, in bits) is defined by each Profile for each
Rule ID.
When there is no DTag, there can be only one fragmented SCHC
Packet in transit for a given Rule ID.
If present, DTag
* MUST be set to the same value for all the SCHC F/R messages
related to the same fragmented SCHC Packet,
* MUST be set to different values for SCHC F/R messages related
to different SCHC Packets that are being fragmented under the
same Rule ID and that may overlap during the fragmented
transmission.
A sequence counter that is incremented for each new fragmented
SCHC Packet, counting from 0 to up to (2^T)-1 and wrapping back to
0 is RECOMMENDED for maximum traceability and replay avoidance.
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o W: The W field is optional. It is only present if windows are
used. Its presence and size (called M, in bits) is defined by
each SCHC F/R mode and each Profile for each Rule ID.
This field carries information pertaining to the window a SCHC F/R
message relates to. If present, W MUST carry the same value for
all the SCHC F/R messages related to the same window. Depending
on the mode and Profile, W may carry the full window number, or
just the least significant bit or any other partial representation
of the window number.
o Fragment Compressed Number (FCN). The FCN field is present in the
SCHC Fragment Header. Its size (called N, in bits) is defined by
each Profile for each Rule ID.
This field conveys information about the progress in the sequence
of tiles being transmitted by SCHC Fragment messages. For
example, it can contain a partial, efficient representation of a
larger-sized tile number. The description of the exact use of the
FCN field is left to each SCHC F/R mode. However, two values are
reserved for special purposes. They help control the SCHC F/R
process:
* The FCN value with all the bits equal to 1 (called All-1)
signals the very last tile of a SCHC Packet. By extension, if
windows are used, the last window of a packet is called the
All-1 window.
* If windows are used, the FCN value with all the bits equal to 0
(called All-0) signals the last tile of a window that is not
the last one of the SCHC packet. By extension, such a window
is called an All-0 window.
The highest value of FCN (an unsigned integer) is called
MAX_WIND_FCN. Since All-1 is reserved, MAX_WIND_FCN MUST be
stricly less that (2^N)-1.
o Message Integrity Check (MIC). This field only appears in the
All-1 SCHC Fragments. Its size (called T, in bits) is defined by
each Profile for each Rule ID.
See Section 8.2.3 for the MIC default size, default polynomials
and details on its computation.
o C (integrity Check): C is a 1-bit field. This field is used in
the SCHC ACK message to report on the reassembled SCHC Packet
integrity check (see Section 8.2.3).
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A value of 1 tells that the integrity check was performed and is
successful. A value of 0 tells that the integrity check was not
performed, or that is was a failure.
o Compressed Bitmap. The Compressed Bitmap is used together with
windows and Bitmaps (see Section 8.2.2.3). Its presence and size
is defined for each F/R mode for each Rule ID.
This field appears in the SCHC ACK message to report on the
receiver Bitmap (see Section 8.3.2.1).
8.3. SCHC F/R Message Formats
This section defines the SCHC Fragment formats, the SCHC ACK format,
the SCHC ACK REQ format and the SCHC Abort formats.
8.3.1. SCHC Fragment format
A SCHC Fragment conforms to the general format shown in Figure 10.
It comprises a SCHC Fragment Header and a SCHC Fragment Payload. The
SCHC Fragment Payload carries one or several tile(s).
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
| Fragment Header | Fragment Payload | padding (as needed)
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
Figure 10: SCHC Fragment general format. Presence of a padding field
is optional
8.3.1.1. Regular SCHC Fragment
The Regular SCHC Fragment format is shown in Figure 11. Regular SCHC
Fragments are generally used to carry tiles that are not the last one
of a SCHC Packet. The DTag field and the W field are optional.
|--- SCHC Fragment Header ----|
|-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | FCN | Fragment Payload | padding (as needed)
+-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
Figure 11: Detailed Header Format for Regular SCHC Fragments
The FCN field MUST NOT contain all bits set to 1.
If the size of the SCHC Fragment Payload does not nicely complement
the SCHC Header size in a way that would make the SCHC Fragment a
multiple of the L2 Word, then padding bits MUST be added.
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The Fragment Payload of a SCHC Fragment with FCN == 0 (called an
All-0 SCHC Fragment) MUST be at least the size of an L2 Word. The
rationale is that, even in the presence of padding, an All-0 SCHC
Fragment needs to be distinguishable from the SCHC ACK REQ message,
which has the same header but has no payload (see Section 8.3.3).
8.3.1.2. All-1 SCHC Fragment
The All-1 SCHC Fragment format is shown in Figure 12. The All-1 SCHC
Fragment is generally used to carry the very last tile of a SCHC
Packet and a MIC, or a MIC only. The DTag field, the W field and the
Payload are optional.
|-------- SCHC Fragment Header -------|
|-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 11..1 | MIC | Frag Payload | pad. (as needed)
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
(FCN)
Figure 12: Detailed format for the All-1 SCHC Fragment
If the size of the SCHC Fragment Payload does not nicely complement
the SCHC Header size in a way that would make the SCHC Fragment a
multiple of the L2 Word, then padding bits MUST be added.
The All-1 SCHC Fragment message MUST be distinguishable by size from
a SCHC Sender-Abort message (see Section 8.3.4.1) that has the same
T, M and N values. This is trivially achieved by having the MIC
larger than an L2 Word, or by having the Payload larger than an L2
Word. This is also naturally achieved if the SCHC Sender-Abort
Header is a multiple of L2 Words.
8.3.2. SCHC ACK format
The SCHC ACK message MUST obey the format shown in Figure 13. The
DTag field, the W field and the Compressed Bitmap field are optional.
The Compressed Bitmap field can only be present in SCHC F/R modes
that use windows.
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|---- SCHC ACK Header ----|
|-- T --|-M-|1|
+---- ... --+- ... -+---+-+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W |1| padding as needed (success)
+---- ... --+- ... -+---+-+~~~~~~~~~~~~~~~~~~
+---- ... --+- ... -+---+-+------ ... ------+~~~~~~~~~~~~~~~
| Rule ID | DTag | W |0|Compressed Bitmap| pad. as needed (failure)
+---- ... --+- ... -+---+-+------ ... ------+~~~~~~~~~~~~~~~
C
Figure 13: Format of the SCHC ACK message
The SCHC ACK Header contains a C bit (see Section 8.2.4).
If the C bit is set to 1 (integrity check successful), no Bitmap is
carried and padding bits MUST be appended as needed to fill up the
last L2 Word.
If the C bit is set to 0 (integrity check not performed or failed)
and if windows are used,
o a representation of the Bitmap for the window referred to by the W
field MUST follow the C bit
o padding bits MUST be appended as needed to fill up the last L2
Word
If the C bit is 1 or windows are not used, the C bit MUST be followed
by padding bits as needed to fill up the last L2 Word.
See Section 8.2.2.3 for a description of the Bitmap.
The representation of the Bitmap that is transmitted MUST be the
compressed version specified in Section 8.3.2.1, in order to reduce
the SCHC ACK message size.
8.3.2.1. Bitmap Compression
For transmission, the Compressed Bitmap in the SCHC ACK message is
defined by the following algorithm (see Figure 14 for a follow-along
example):
o Build a temporary SCHC ACK message that contains the Header
followed by the original Bitmap.
o Positioning scissors at the end of the Bitmap, after its last bit.
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o While the bit on the left of the scissors is 1 and belongs to the
Bitmap, keep moving left, then stop. When this is done,
o While the scissors are not on an L2 Word boundary of the SCHC ACK
message and there is a Bitmap bit on the right of the scissors,
keep moving right, then stop.
o At this point, cut and drop off any bits to the right of the
scissors
When one or more bits have effectively been dropped off as a result
of the above algorithm, the SCHC ACK message is a multiple of L2
Words, no padding bits will be appended.
Because the SCHC Fragment sender knows the size of the original
Bitmap, it can reconstruct the original Bitmap from the Compressed
Bitmap received in the SCH ACK message.
Figure 14 shows an example where L2 Words are actually bytes and
where the original Bitmap contains 17 bits, the last 15 of which are
all set to 1.
|---- SCHC ACK Header ----|-------- Bitmap --------|
|-- T --|-M-|1|
+---- ... --+- ... -+---+-+---------------------------------+
| Rule ID | DTag | W |0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+---- ... --+- ... -+---+-+---------------------------------+
C |
next L2 Word boundary ->|
Figure 14: Tentative SCHC ACK message with Bitmap before compression
Figure 15 shows that the last 14 bits are not sent.
|---- SCHC ACK Header ----|CpBmp|
|-- T --|-M-|1|
+---- ... --+- ... -+---+-+-----+
| Rule ID | DTag | W |0|1 0 1|
+---- ... --+- ... -+---+-+-----+
C |
next L2 Word boundary ->|
Figure 15: Actual SCHC ACK message with Compressed Bitmap, no padding
Figure 16 shows an example of a SCHC ACK with tile numbers ranging
from 6 down to 0, where the Bitmap indicates that the second and the
fourth tile of the window have not been correctly received.
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|---- SCHC ACK Header ----|--- Bitmap --|
|-- T --|-M-|1|6 5 4 3 2 1 0| (tile #)
+-----------+-------+---+-+-------------+
| Rule ID | DTag | W |0|1 0 1 0 1 1 1| with Original Bitmap
+-----------+-------+---+-+-------------+
C
next L2 Word boundary ->||
+-----------+-------+---+-+-------------+~~~+
| Rule ID | DTag | W |0|1 0 1 0 1 1 1|Pad| transmitted SCHC ACK
+-----------+-------+---+-+-------------+~~~+
C
next L2 Word boundary ->||
Figure 16: Example of a SCHC ACK message, missing tiles, with padding
Figure 17 shows an example of a SCHC ACK with FCN ranging from 6 down
to 0, where integrity check has not been performed or has failed and
the Bitmap indicates that there is no missing tile in that window.
|---- SCHC ACK Header ----|--- Bitmap --|
|-- T --|-M-|1|6 5 4 3 2 1 0| (tile #)
+-----------+-------+---+-+-------------+
| Rule ID | DTag | W |0|1 1 1 1 1 1 1| with Original Bitmap
+-----------+-------+---+-+-------------+
C
next L2 Word boundary ->|
+---- ... --+- ... -+---+-+-+
| Rule ID | DTag | W |0|1| transmitted SCHC ACK
+---- ... --+- ... -+---+-+-+
C
next L2 Word boundary ->|
Figure 17: Example of a SCHC ACK message, no missing tile, no padding
8.3.3. SCHC ACK REQ format
The SCHC ACK REQ is used by a sender to explicitely request a SCHC
ACK from the receiver. Its format is described in Figure 18. The
DTag field and the W field are optional.
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|---- SCHC ACK REQ Header ----|
|-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 0..0 | padding (as needed) (no payload)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 18: SCHC ACK REQ detailed format
The size of the SCHC ACK REQ header is generally not a multiple of
the L2 Word size. Therefore, a SCHC ACK REQ generally needs padding
bits.
Note that the SCHC ACK REQ has the same header as an All-0 SCHC
Fragment (see Section 8.3.1.1) but it doesn't have a payload. A
receiver can distinguish the former form the latter by the message
length, even in the presence of padding. This is possible because
o the padding bits are always stricly less than an L2 Word.
o the size of an All-0 SCHC Fragment Payload is at least the size of
an L2 Word,
8.3.4. SCHC Abort formats
8.3.4.1. SCHC Sender-Abort
When a SCHC Fragment sender needs to abort an on-going fragmented
SCHC Packet transmission, it sends a SCHC Sender-Abort message to the
SCHC Fragment receiver.
The SCHC Sender-Abort format is described in Figure 19. The DTag
field and the W field are optional.
|---- Sender-Abort Header ----|
|-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 11..1 | padding (as needed)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 19: SCHC Sender-Abort format
If the W field is present,
o the fragment sender MUST set it to all 1's. Other values are
RESERVED.
o the fragment receiver MUST check its value. If the value is
different from all 1's, the message MUST be ignored.
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The size of the SCHC Sender-Abort header is generally not a multiple
of the L2 Word size. Therefore, a SCHC Sender-Abort generally needs
padding bits.
Note that the SCHC Sender-Abort has the same header as an All-1 SCHC
Fragment (see Section 8.3.1.2), but that it does not include a MIC
nor a payload. The receiver distinguishes the former from the latter
by the message length, even in the presence of padding. This is
possible through different combinations
o the size of the Sender-Abort Header may be made such that it is
not padded
o or the total size of the MIC and the Payload of an All-1 SCHC
Fragment is at least the size of an L2 Word
o or through other alignment and size combinations
The SCHC Sender-Abort MUST NOT be acknowledged.
8.3.4.2. SCHC Receiver-Abort
When a SCHC Fragment receiver needs to abort an on-going fragmented
SCHC Packet transmission, it transmits a SCHC Receiver-Abort message
to the SCHC Fragment sender.
The SCHC Receiver-Abort format is described in Figure 20. The DTag
field and the W field are optional.
|--- Receiver-Abort Header ---|
|--- T ---|-M-|1|
+---- ... ----+-- ... --+---+-+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag | W |1| 1..1| 1..1 |
+---- ... ----+-- ... --+---+-+-+-+-+-+-+-+-+-+-+-+-+
C
next L2 Word boundary ->||
Figure 20: SCHC Receiver-Abort format
If the W field is present,
o the fragment receiver MUST set it to all 1's. Other values are
RESERVED.
o the fragment sender MUST check its value. If the value is
different from all 1's, the message MUST be ignored.
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Note that the SCHC Receiver-Abort has the same header as a SCHC ACK
message. The bits that follow the SCHC Receiver-Abort Header MUST be
as follows
o if the Header does not end at an L2 Word boundary, append bits set
to 1 as needed to reach the next L2 Word boundary
o append exactly one more L2 Word with bits all set to 1's
Such a bit pattern never occurs in a regular SCHC ACK. This is how
the fragment sender recognizes a SCHC Receiver-Abort.
A SCHC Receiver-Abort is aligned to L2 Words, by design. Therefore,
padding MUST NOT be appended.
The SCHC Receiver-Abort MUST NOT be acknowledged.
8.4. SCHC F/R modes
This specification includes several SCHC F/R modes, which allow for
o a range of reliability options, such as optional SCHC Fragment
retransmission
o support of different LPWAN characteristics, such as variable MTU.
More modes may be defined in the future.
8.4.1. No-ACK mode
The No-ACK mode has been designed under the assumption that data unit
out-of-sequence delivery does not occur between the entity performing
fragmentation and the entity performing reassembly. This mode
supports LPWAN technologies that have a variable MTU.
In No-ACK mode, there is no feedback communication from the fragment
receiver to the fragment sender. The sender just transmits all the
SCHC Fragments blindly.
Padding is kept to a minimum: only the last SCHC Fragment is padded
as needed.
The tile sizes are not required to be uniform. Windows are not used.
The Retransmission Timer is not used. The Attempts counter is not
used.
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Each Profile MUST specify which Rule ID value(s) is (are) allocated
to this mode. For brevity, the rest of Section 8.4.1 only refers to
Rule ID values that are allocated to this mode.
The W field MUST NOT be present in the SCHC F/R messages. SCHC ACK
MUST NOT be sent. SCHC ACK REQ MUST NOT be sent. SCHC Sender-Abort
MAY be sent. SCHC Receiver-Abort MUST NOT be sent.
The value of N (size of the FCN field) is RECOMMENDED to be 1.
Each Profile, for each Rule ID value, MUST define
o the presence or absence of the DTag field in the SCHC F/R
messages, as well as its size if it is present,
o the size and algorithm for the MIC field in the SCHC F/R messages,
if different from the default,
o the expiration time of the Inactivity Timer
Each Profile, for each Rule ID value, MAY define
o a value of N different from the recommend one,
o what values will be sent in the FCN field, for values different
from the All-1 value.
The receiver, for each pair of Rule ID and optional DTag values, MUST
maintain
o one Inactivity Timer
8.4.1.1. Sender behaviour
At the beginning of the fragmentation of a new SCHC Packet, the
fragment sender MUST select a Rule ID and optional DTag value pair
for this SCHC Packet. For brevity, the rest of Section 8.4.1 only
refers to SCHC F/R messages bearing the Rule ID and optional DTag
values hereby selected.
Each SCHC Fragment MUST contain exactly one tile in its Payload. The
tile MUST be at least the size of an L2 Word. The sender MUST
transmit the SCHC Fragments messages in the order that the tiles
appear in the SCHC Packet. Except for the last tile of a SCHC
Packet, each tile MUST be of a size that complements the SCHC
Fragment Header so that the SCHC Fragment is a multiple of L2 Words
without the need for padding bits. Except for the last one, the SCHC
Fragments MUST use the Regular SCHC Fragment format specified in
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Section 8.3.1.1. The last SCHC Fragment MUST use the All-1 format
specified in Section 8.3.1.2.
The MIC MUST be computed on the reassembled SCHC Packet concatenated
with the padding bits of the last SCHC Fragment. The rationale is
that the SCHC Reassembler has no way of knowing where the payload of
the last SCHC Fragment ends. Indeed, this requires decompressing the
SCHC Packet, which is out of the scope of the SCHC Reassembler.
The sender MAY transmit a SCHC Sender-Abort.
Figure 35 shows an example of a corresponding state machine.
8.4.1.2. Receiver behaviour
On receiving Regular SCHC Fragments,
o the receiver MUST reset the Inactivity Timer,
o the receiver assembles the payloads of the SCHC Fragments
On receiving an All-1 SCHC Fragment,
o the receiver MUST append the All-1 SCHC Fragment Payload and the
padding bits to the previously received SCHC Fragment Payloads for
this SCHC Packet
o if an integrity checking is specified in the Profile,
* the receiver MUST perform the integrity check
* if integrity checking fails, the receiver MUST drop the
reassembled SCHC Packet and it MUST release all resources
associated with this Rule ID and optional DTag values.
o the reassembly operation concludes.
On expiration of the Inactivity Timer, the receiver MUST drop the
SCHC Packet being reassembled and it MUST release all resources
associated with this Rule ID and optional DTag values.
On receiving a SCHC Sender-Abort, the receiver MAY release all
resources associated with this Rule ID and optional DTag values.
The MIC computed at the receiver MUST be computed over the
reassembled SCHC Packet and over the padding bits that were received
in the SCHC Fragment carrying the last tile.
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Figure 36 shows an example of a corresponding state machine.
8.4.2. ACK-Always
The ACK-Always mode has been designed under the following assumptions
o Data unit out-of-sequence delivery does not occur between the
entity performing fragmentation and the entity performing
reassembly
o The L2 MTU value does not change while a fragmented SCHC Packet is
being transmitted.
In ACK-Always mode, windows are used. An acknowledgement, positive
or negative, is fed by the fragment receiver back to the fragment
sender at the end of the transmission of each window of SCHC
Fragments.
The tiles are not required to be of uniform size. Padding is kept to
a minimum: only the last SCHC Fragment is padded as needed.
In a nutshell, the algorithm is the following: after a first blind
transmission of all the tiles of a window, the fragment sender
iterates retransmitting the tiles that are reported missing until the
fragment receiver reports that all the tiles belonging to the window
have been correctly received, or until too many attempts were made.
The fragment sender only advances to the next window of tiles when it
has ascertained that all the tiles belonging to the current window
have been fully and correctly received. This results in a lock-step
behaviour between the sender and the receiver, at the window
granularity.
Each Profile MUST specify which Rule ID value(s) is (are) allocated
to this mode. For brevity, the rest of Section 8.4.1 only refers to
Rule ID values that are allocated to this mode.
The W field MUST be present and its size M MUST be 1 bit.
WINDOW_SIZE MUST be equal to MAX_WIND_FCN + 1.
Each Profile, for each Rule ID value, MUST define
o the value of N (size of the FCN field),
o the value of MAX_WIND_FCN
o the size and algorithm for the MIC field in the SCHC F/R messages,
if different from the default,
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o the presence or absence of the DTag field in the SCHC F/R
messages, as well as its size if it is present,
o the value of MAX_ACK_REQUESTS,
o the expiration time of the Retransmission Timer
o the expiration time of the Inactivity Timer
The sender, for each active pair of Rule ID and optional DTag values,
MUST maintain
o one Attempts counter
o one Retransmission Timer
The receiver, for each pair of Rule ID and optional DTag values, MUST
maintain
o one Inactivity Timer
8.4.2.1. Sender behaviour
At the beginning of the fragmentation of a new SCHC Packet, the
fragment sender MUST select a Rule ID and DTag value pair for this
SCHC Packet. For brevity, the rest of Section 8.4.2 only refers to
SCHC F/R messages bearing the Rule ID and optional DTag values hereby
selected.
Each SCHC Fragment MUST contain exactly one tile in its Payload. All
tiles with the number 0 in their window, as well as the last tile,
MUST be at least the size of an L2 Word.
In all SCHC Fragment messages, the W field MUST be filled with the
least significant bit of the window number that the sender is
currently processing.
If a SCHC Fragment carries a tile that is not the last one of the
SCHC Packet,
o it MUST be of the Regular type specified in Section 8.3.1.1
o the FCN field MUST contain the tile number
o each tile MUST be of a size that complements the SCHC Fragment
Header so that the SCHC Fragment is a multiple of L2 Words without
the need for padding bits.
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The SCHC Fragment that carries the last tile MUST be an All-1 SCHC
Fragment, described in Section 8.3.1.2.
The bits on which the MIC is computed MUST be the SCHC Packet
concatenated with the potential padding bits that are appended to the
Payload of the SCHC Fragment that carries the last tile.
The fragment sender MUST start by processing the window numbered 0.
In a "blind transmission" phase, it MUST transmit all the tiles
composing the window, in decreasing tile number.
Then, it enters an "equalization phase" in which it MUST initialize
an Attempts counter to 0, it MUST start a Retransmission Timer and it
MUST expect to receive a SCHC ACK. Then,
o on receiving a SCHC ACK,
* if the SCHC ACK indicates that some tiles are missing at the
receiver, then the sender MUST transmit all the tiles that have
been reported missing, it MUST increment Attempts, it MUST
reset the Retransmission Timer and MUST expect to receive a
SCHC ACK again.
* if the current window is not the last one and the SCHC ACK
indicates that all tiles were correctly received, the sender
MUST stop the Retransmission Timer, it MUST advance to the next
fragmentation window and it MUST start a blind transmission
phase as described above.
* if the current window is the last one and the SCHC ACK
indicates that more tiles were received than the sender
actually sent, the fragment sender MUST send a SCHC Sender-
Abort, it MUST release all resource associated with this SCHC
Packet and it MAY exit with an error condition.
* if the current window is the last one and the SCHC ACK
indicates that all tiles were correctly received yet integrity
check was a failure, the fragment sender MUST send a SCHC
Sender-Abort, it MUST release all resource associated with this
SCHC Packet and it MAY exit with an error condition.
* if the current window is the last one and the SCHC ACK
indicates that integrity checking was successful, the sender
exits successfully.
o on Retransmission Timer expiration,
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* if Attempts is strictly less that MAX_ACK_REQUESTS, the
fragment sender MUST send a SCHC ACK REQ and MUST increment the
Attempts counter.
* otherwise the fragment sender MUST send a SCHC Sender-Abort, it
MUST release all resource associated with this SCHC Packet and
it MAY exit with an error condition.
At any time,
o on receiving a SCHC Receiver-Abort, the fragment sender MUST
release all resource associated with this SCHC Packet and it MAY
exit with an error condition.
o on receiving a SCHC ACK that bears a W value different from the W
value that it currently uses, the fragment sender MUST silently
discard and ignore that SCHC ACK.
Figure 37 shows an example of a corresponding state machine.
8.4.2.2. Receiver behaviour
On receiving a SCHC Fragment with a Rule ID and optional DTag pair
not being processed at that time
o the receiver MAY check if the optional DTag value has not recently
been used for that Rule ID value, thereby ensuring that the
received SCHC Fragment is not a remnant of a prior fragmented SCHC
Packet transmission. If the SCHC Fragment is determined to be
such a remant, the receiver MAY silently ignore it and discard it.
o the receiver MUST start a process to assemble a new SCHC Packet
with that Rule ID and DTag value pair. That process MUST only
examine received SCHC F/R messages with that Rule ID and DTag
value pair and MUST only transmit SCHC F/R messages with that Rule
ID and DTag value pair.
o the receiver MUST start an Inactivity Timer. It MUST initialise
an Attempts counter to 0. It MUST initialise a window counter to
0.
In the rest of this section, "local W bit" means the least
significant bit of the window counter of the receiver.
On reception of any SCHC F/R message, the receiver MUST reset the
Inactivity Timer.
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Entering an "acceptance phase", the receiver MUST first initialise an
empty Bitmap for this window, then
o on receiving a SCHC Fragment or SCHC ACK REQ with the W bit
different from the local W bit, the receiver MUST silently ignore
and discard that message.
o on receiving a SCHC Fragment with the W bit equal to the local W
bit, the receiver MUST assemble the received tile based on the
window counter and on the FCN field in the SCHC Fragment and it
MUST update the Bitmap.
* if the SCHC Fragment received is an All-0 SCHC Fragment, the
current window is determined to be a not-last window, and the
receiver MUST send a SCHC ACK for this window. Then,
+ If the Bitmap indicates that all the tiles of the current
window have been correctly received, the receiver MUST
increment its window counter and it enters the "acceptance
phase" for that new window.
+ If the Bitmap indicates that at least one tile is missing in
the current window, the receiver enters the "equalization
phase" for this window.
* if the SCHC Fragment received is an All-1 SCHC Fragment, the
padding bits of the All-1 SCHC Fragment MUST be assembled after
the received tile, the current window is determined to be the
last window, the receiver MUST perform the integrity check and
it MUST send a SCHC ACK for this window. Then,
+ If the integrity check indicates that the full SCHC Packet
has been correctly reassembled, the receiver MUST enter the
"clean-up phase".
+ If the integrity check indicates that the full SCHC Packet
has not been correctly reassembled, the receiver enters the
"equalization phase" for this window.
o on receiving a SCHC ACK REQ with the W bit equal to the local W
bit, the receiver has not yet determined if the current window is
a not-last one or the last one, the receiver MUST send a SCHC ACK
for this window, and it keeps accepting incoming messages.
In the "equalization phase":
o if the window is a not-last window
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* on receiving a SCHC Fragment or SCHC ACK REQ with a W bit
different from the local W bit the receiver MUST silently
ignore and discard that message.
* on receiving a SCHC ACK REQ with a W bit equal to the local W
bit, the receiver MUST send a SCHC ACK for this window.
* on receiving a SCHC Fragment with a W bit equal to the local W
bit,
+ if the SCHC Fragment received is an All-1 SCHC Fragment, the
receiver MUST silently ignore it and discard it.
+ otherwise, the receiver MUST update the Bitmap and it MUST
assemble the tile received.
* on the Bitmap becoming fully populated with 1's, the receiver
MUST send a SCHC ACK for this window, it MUST increment its
window counter and it enters the "acceptance phase" for the new
window.
o if the window is the last window
* on receiving a SCHC Fragment or SCHC ACK REQ with a W bit
different from the local W bit the receiver MUST silently
ignore and discard that message.
* on receiving a SCHC ACK REQ with a W bit equal to the local W
bit, the receiver MUST send a SCHC ACK for this window.
* on receiving a SCHC Fragment with a W bit equal to the local W
bit,
+ if the SCHC Fragment received is an All-0 SCHC Fragment, the
receiver MUST silently ignore it and discard it.
+ otherwise, the receiver MUST update the Bitmap and it MUST
assemble the tile received. If the SCHC Fragment received
is an All-1 SCHC Fragment, the receiver MUST assemble the
padding bits of the All-1 SCHC Fragment after the received
tile. It MUST perform the integrity check. Then
- if the integrity check indicates that the full SCHC
Packet has been correctly reassembled, the receiver MUST
send a SCHC ACK and it enters the "clean-up phase".
- if the integrity check indicates that the full SCHC
Packet has not been correctly reassembled,
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o if the SCHC Fragment received was an All-1 SCHC
Fragment, the receiver MUST send a SCHC ACK for this
window
o it keeps accepting incoming messages.
In the "clean-up phase":
o Any received SCHC F/R message with a W bit different from the
local W bit MUST be silently ignored and discarded.
o Any received SCHC F/R message different from an All-1 SCHC
Fragment or a SCHC ACK REQ MUST be silently ignored and discarded.
o On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, the
receiver MUST send a SCHC ACK.
o On expiration of the Inactivity Timer, the receive process for
that SCHC Packet MAY exit
At any time, on expiration of the Inactivity Timer, on receiving a
SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the
receiver MUST send a SCHC Receiver-Abort, it MUST release all
resource associated with this SCHC Packet and it MAY exit the receive
process for that SCHC Packet.
The MIC computed at the receiver MUST be computed over the
reassembled SCHC Packet and over the padding bits that were received
in the SCHC Fragment carrying the last tile.
Figure 38 shows an example of a corresponding state machine.
8.4.3. ACK-on-Error
The ACK-on-Error mode supports LPWAN technologies that have variable
MTU and out-of-order delivery.
In ACK-on-Error mode, windows are used. All tiles MUST be of equal
size, except for the last one, which MUST be of the same size or
smaller than the preceding ones. WINDOW_SIZE MUST be equal to
MAX_WIND_FCN + 1.
A SCHC Fragment message carries one or more tiles, which may span
multiple windows. A SCHC ACK reports on the reception of exactly one
window of tiles.
See Figure 21 for an example.
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+---------------------------------------------...-----------+
| SCHC Packet |
+---------------------------------------------...-----------+
Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 |3|
Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|- 28-|
SCHC Fragment msg |-----------|
Figure 21: a SCHC Packet fragmented in tiles, Ack-on-Error mode
The W field is wide enough that it unambiguously represents an
absolute window number. The fragment receiver feeds SCHC ACKs back
to the fragment sender about windows that it misses tiles of. No
SCHC ACK is fed back by the fragment receiver for windows that it
knows have been fully received.
The fragment sender retransmits SCHC Fragments for tiles that are
reported missing. It can advance to next windows even before it has
ascertained that all tiles belonging to previous windows have been
correctly received, and can still later retransmit SCHC Fragments
with tiles belonging to previous windows. Therefore, the sender and
the receiver may operate in a fully decoupled fashion. The
fragmented SCHC Packet transmission concludes when
o integrity checking shows that the fragmented SCHC Packet has been
correctly reassembled at the receive end, and this information has
been conveyed back to the sender,
o or too many retransmission attempts were made,
o or the receiver determines that the transmission of this
fragmented SCHC Packet has been inactive for too long.
Each Profile MUST specify which Rule ID value(s) is (are) allocated
to this ACK-on-Error mode. For brevity, the rest of Section 8.4.3
only refers to SCHC F/R messages with Rule ID values that are
allocated to this mode.
The W field MUST be present in the SCHC F/R messages.
Each Profile, for each Rule ID value, MUST define
o the tile size (a tile does not need to be multiple of an L2 Word,
but it MUST be at least the size of an L2 Word)
o the value of M (size of the W field),
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o the value of N (size of the FCN field),
o the value of MAX_WIND_FCN
o the size and algorithm for the MIC field in the SCHC F/R messages,
if different from the default,
o the presence or absence of the DTag field in the SCHC F/R
messages, as well as its size if it is present,
o the value of MAX_ACK_REQUESTS,
o the expiration time of the Retransmission Timer
o the expiration time of the Inactivity Timer
The sender, for each active pair of Rule ID and optional DTag values,
MUST maintain
o one Attempts counter
o one Retransmission Timer
The receiver, for each pair of Rule ID and optional DTag values, MUST
maintain
o one Inactivity Timer
8.4.3.1. Sender behaviour
At the beginning of the fragmentation of a new SCHC Packet,
o the fragment sender MUST select a Rule ID and DTag value pair for
this SCHC Packet. A Rule MUST NOT be selected if the values of M
and MAX_WIND_FCN for that Rule are such that the SCHC Packet
cannot be fragmented in (2ˆM) * (MAX_WIND_FCN+1) tiles or
less.
o the fragment sender MUST initialize the Attempts counter to 0 for
that Rule ID and DTag value pair.
For brevity, the rest of Section 8.4.3 only refers to SCHC F/R
messages bearing the Rule ID and optional DTag values hereby
selected.
A SCHC Fragment message carries in its payload one or more tiles. If
more than one tile is carried in one SCHC Fragment
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o the selected tiles MUST be consecutive in the original SCHC Packet
o they MUST be placed in the SCHC Fragment Payload adjacent to one
another, in the order they appear in the SCHC Packet, from the
start of the SCHC Packet toward its end.
In a SCHC Fragment message, the sender MUST fill the W field with the
window number of the first tile sent in that SCHC Fragment.
If a SCHC Fragment carries more than one tile, or carries one tile
that is not the last one of the SCHC Packet,
o it MUST be of the Regular type specified in Section 8.3.1.1
o the FCN field MUST contain the tile number of the first tile sent
in that SCHC Fragment
o padding bits are appended to the tiles as needed to fit the
Payload size constraint of Regular SCHC Fragments
The bits on which the MIC is computed MUST be the SCHC Packet
concatenated with the padding bits that are appended to the Payload
of the SCHC Fragment that carries the last tile.
The fragment sender MAY send the last tile as the Payload of an All-1
SCHC Fragment.
The fragment sender MUST send SCHC Fragments such that, all together,
they contain all the tiles of the fragmented SCHC Packet.
The fragment sender MUST send at least one All-1 SCHC Fragment.
Note that the last tile of a SCHC Packet can be sent in different
ways, depending on Profiles and implementations
o in a Regular SCHC Fragment, either alone or as part of multiple
tiles Payload
o in an All-1 SCHC Fragment
However, the last tile MUST NOT have ever been sent both in a Regular
SCHC Fragment and in a All-1 SCHC Fragment.
The fragment sender MUST listen for SCHC ACK messages after having
sent
o an All-1 SCHC Fragment
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o or a SCHC ACK REQ with the W field corresponding to the last
window.
A Profile MAY specify other times at which the fragment sender MUST
listen for SCHC ACK messages.
Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC
ACK REQ,
o it MUST increment the Attempts counter
o it MUST reset the Retransmission Timer
On Retransmission Timer expiration
o if Attempts is strictly less than MAX_ACK_REQUESTS, the fragment
sender MUST send a SCHC ACK REQ with the W field corresponding to
the last window and it MUST increment the Attempts counter
o otherwise the fragment sender MUST send a SCHC Sender-Abort and it
MUST release all resource associated with this SCHC Packet.
On receiving a SCHC ACK,
o if the W field in the SCHC ACK corresponds to the last window of
the SCHC Packet,
* if the C bit is set, the sender MAY release all resource
associated with this SCHC Packet and MAY exit successfully
* otherwise,
+ if the SCHC ACK shows no missing tile at the receiver, the
sender
- MUST send a SCHC Sender-Abort
- MUST release all resource associated with this SCHC
Packet
- MAY exit with an error condition
+ otherwise
- the fragment sender MUST send SCHC Fragment messages
containing all the tiles that are reported missing in the
SCHC ACK.
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- if the last message in this sequence of SCHC Fragment
messages is not an All-1 SCHC Fragment, then the fragment
sender MUST send a SCHC ACK REQ with the W field
corresponding to the last window after the sequence.
o otherwise, the fragment sender
* MUST send SCHC Fragment messages containing the tiles that are
reported missing in the SCHC ACK
* then it MAY send a SCHC ACK REQ with the W field corresponding
to the last window
See Figure 39 for one among several possible examples of a Finite
State Machine implementing a sender behaviour obeying this
specification.
8.4.3.2. Receiver behaviour
On receiving a SCHC Fragment with a Rule ID and optional DTag pair
not being processed at that time
o the receiver MAY check if the optional DTag value has not recently
been used for that Rule ID value, thereby ensuring that the
received SCHC Fragment is not a remnant of a prior fragmented SCHC
Packet transmission. If the SCHC Fragment is determined to be
such a remant, the receiver MAY silently ignore it and discard it.
o the receiver MUST start a process to assemble a new SCHC Packet
with that Rule ID and DTag value pair. That process MUST only
examine received SCHC F/R messages with that Rule ID and DTag
value pair and MUST only transmit SCHC F/R messages with that Rule
ID and DTag value pair.
o the receiver MUST start an Inactivity Timer. It MUST initialise
an Attempts counter to 0.
On reception of any SCHC F/R message, the receiver MUST reset the
Inactivity Timer.
On reception of a SCHC Fragment message, the receiver MUST assemble
the received tiles based on the W and FCN fields of the SCHC
Fragment.
o if the FCN is All-1, if a Payload is present, the full SCHC
Fragment Payload MUST be assembled including the padding bits.
This is because the size of the last tile is not known by the
receiver, therefore padding bits are indistinguishable from the
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tile data bits, at this stage. They will be removed by the SCHC
C/D sublayer. If the size of the SCHC Fragment Payload exceeds or
equals the size of one regular tile plus the size of an L2 Word,
this SHOULD raise an error flag.
o otherwise, tiles MUST be assembled based on the a priori known
size and padding bits MUST be discarded. The latter is possible
because
* the size of the tiles is known a priori,
* tiles are larger than an L2 Word
* padding bits are always strictly less than an L2 Word
On reception of a SCHC ACK REQ or of an All-1 SCHC Fragment,
o if the receiver has at least one window that it knows has tiles
missing, it MUST return a SCHC ACK for the lowest-numbered such
window,
o otherwise,
* if it has received at least one tile, it MUST return a SCHC ACK
for the highest-numbered window it currently has tiles for
* otherwise it MUST return a SCHC ACK for window numbered 0
A Profile MAY specify other times and circumstances at which a
receiver sends a SCHC ACK, and which window the SCHC ACK reports
about in these circumstances.
On sending a SCHC ACK, the receiver MUST increase the Attempts
counter.
From reception of an All-1 SCHC Fragment onward, a receiver MUST
check the integrity of the reassembled SCHC Packet at least every
time it prepares for sending a SCHC ACK for the last window.
On reception of a SCHC Sender-Abort, the receiver MUST release all
resource associated with this SCHC Packet.
On expiration of the Inactivity Timer, the receiver MUST send a SCHC
Receiver-Abort and it MUST release all resource associated with this
SCHC Packet.
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On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST
send a SCHC Receiver-Abort and it MUST release all resource
associated with this SCHC Packet.
Reassembly of the SCHC Packet concludes when
o a Sender-Abort has been received
o or the Inactivity Timer has expired
o or the Attempts counter has exceeded MAX_ACK_REQUESTS
o or when at least an All-1 SCHC Fragment has been received and
integrity checking of the reassembled SCHC Packet is successful.
The MIC computed at the receiver MUST be computed over the
reassembled SCHC Packet and over the padding bits that were received
in the SCHC Fragment carrying the last tile.
See Figure 40 for one among several possible examples of a Finite
State Machine implementing a receiver behaviour obeying this
specification, and that is meant to match the sender Finite State
Machine of Figure 39.
9. Padding management
SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does
not have any alignment prerequisite. The size of SCHC Packets can be
any number of bits. If the layer below SCHC constrains the payload
to align to some boundary, called L2 Words (for example, bytes), SCHC
will meet that constraint and produce messages with the correct
alignement. This may entail adding extra bits, called padding bits.
When padding occurs, the number of appended bits MUST be strictly
less than the L2 Word size.
Padding happens at most once for each Packet during SCHC Compression
and optional SCHC Fragmentation (see Figure 2). If a SCHC Packet is
sent unfragmented (see Figure 22), it is padded as needed for
transmission. If a SCHC Packet is fragmented, it is not padded in
itself, only the SCHC Fragments are padded as needed for
transmission. Some SCHC F/R modes only pad the very last SCHC
Fragment.
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A packet (e.g. an IPv6 packet)
| ^ (padding bits
v | dropped)
+------------------+ +--------------------+
| SCHC Compression | | SCHC Decompression |
+------------------+ +--------------------+
| ^
| If no fragmentation |
+---- SCHC Packet + padding as needed ----->|
| | (MIC checked
v | and removed)
+--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+
| ^ | ^
| | | |
| +------------- SCHC ACK ------------+ |
| |
+------- SCHC Fragments + padding as needed---------+
SENDER RECEIVER
Figure 22: SCHC operations, including padding as needed
Each Profile MUST specify the size of the L2 Word. The L2 Word might
actually be a single bit, in which case at most zero bits of padding
will be appended to any message, i.e. no padding will take place at
all.
A Profile MAY define the value of the padding bits. The RECOMMENDED
value is 0.
10. SCHC Compression for IPv6 and UDP headers
This section lists the different IPv6 and UDP header fields and how
they can be compressed.
10.1. IPv6 version field
This field always holds the same value. Therefore, in the Rule, TV
is set to 6, MO to "equal" and CDA to "not-sent".
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10.2. IPv6 Traffic class field
If the DiffServ field does not vary and is known by both sides, the
Field Descriptor in the Rule SHOULD contain a TV with this well-known
value, an "equal" MO and a "not-sent" CDA.
Otherwise (e.g. ECN bits are to be transmitted), two possibilities
can be considered depending on the variability of the value:
o One possibility is to not compress the field and send the original
value. In the Rule, TV is not set to any particular value, MO is
set to "ignore" and CDA is set to "value-sent".
o If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the Rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x) and
CDA to LSB.
10.3. Flow label field
If the Flow Label field does not vary and is known by both sides, the
Field Descriptor in the Rule SHOULD contain a TV with this well-known
value, an "equal" MO and a "not-sent" CDA.
Otherwise, two possibilities can be considered:
o One possibility is to not compress the field and send the original
value. In the Rule, TV is not set to any particular value, MO is
set to "ignore" and CDA is set to "value-sent".
o If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the Rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x) and
CDA to LSB.
10.4. Payload Length field
This field can be elided for the transmission on the LPWAN network.
The SCHC C/D recomputes the original payload length value. In the
Field Descriptor, TV is not set, MO is set to "ignore" and CDA is
"compute-IPv6-length".
If the payload length needs to be sent and does not need to be coded
in 16 bits, the TV can be set to 0x0000, the MO set to MSB(16-s)
where 's' is the number of bits to code the maximum length, and CDA
is set to LSB.
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10.5. Next Header field
If the Next Header field does not vary and is known by both sides,
the Field Descriptor in the Rule SHOULD contain a TV with this Next
Header value, the MO SHOULD be "equal" and the CDA SHOULD be "not-
sent".
Otherwise, TV is not set in the Field Descriptor, MO is set to
"ignore" and CDA is set to "value-sent". Alternatively, a matching-
list MAY also be used.
10.6. Hop Limit field
The field behavior for this field is different for Uplink and
Downlink. In Uplink, since there is no IP forwarding between the Dev
and the SCHC C/D, the value is relatively constant. On the other
hand, the Downlink value depends of Internet routing and MAY change
more frequently. One neat way of processing this field is to use the
Direction Indicator (DI) to distinguish both directions:
o in the Uplink, elide the field: the TV in the Field Descriptor is
set to the known constant value, the MO is set to "equal" and the
CDA is set to "not-sent".
o in the Downlink, send the value: TV is not set, MO is set to
"ignore" and CDA is set to "value-sent".
10.7. IPv6 addresses fields
As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
long fields; one for the prefix and one for the Interface Identifier
(IID). These fields SHOULD be compressed. To allow for a single
Rule being used for both directions, these values are identified by
their role (DEV or APP) and not by their position in the header
(source or destination).
10.7.1. IPv6 source and destination prefixes
Both ends MUST be synchronized with the appropriate prefixes. For a
specific flow, the source and destination prefixes can be unique and
stored in the context. It can be either a link-local prefix or a
global prefix. In that case, the TV for the source and destination
prefixes contain the values, the MO is set to "equal" and the CDA is
set to "not-sent".
If the Rule is intended to compress packets with different prefix
values, match-mapping SHOULD be used. The different prefixes are
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listed in the TV, the MO is set to "match-mapping" and the CDA is set
to "mapping-sent". See Figure 24
Otherwise, the TV contains the prefix, the MO is set to "equal" and
the CDA is set to "value-sent".
10.7.2. IPv6 source and destination IID
If the DEV or APP IID are based on an LPWAN address, then the IID can
be reconstructed with information coming from the LPWAN header. In
that case, the TV is not set, the MO is set to "ignore" and the CDA
is set to "DevIID" or "AppIID". Note that the LPWAN technology
generally carries a single identifier corresponding to the DEV.
Therefore AppIID cannot be used.
For privacy reasons or if the DEV address is changing over time, a
static value that is not equal to the DEV address SHOULD be used. In
that case, the TV contains the static value, the MO operator is set
to "equal" and the CDA is set to "not-sent". [RFC7217] provides some
methods that MAY be used to derive this static identifier.
If several IIDs are possible, then the TV contains the list of
possible IIDs, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent".
It MAY also happen that the IID variability only expresses itself on
a few bytes. In that case, the TV is set to the stable part of the
IID, the MO is set to "MSB" and the CDA is set to "LSB".
Finally, the IID can be sent in extenso on the LPWAN. In that case,
the TV is not set, the MO is set to "ignore" and the CDA is set to
"value-sent".
10.8. IPv6 extensions
No Rule is currently defined that processes IPv6 extensions. If such
extensions are needed, their compression/decompression Rules can be
based on the MOs and CDAs described above.
10.9. UDP source and destination port
To allow for a single Rule being used for both directions, the UDP
port values are identified by their role (DEV or APP) and not by
their position in the header (source or destination). The SCHC C/D
MUST be aware of the traffic direction (Uplink, Downlink) to select
the appropriate field. The following Rules apply for DEV and APP
port numbers.
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If both ends know the port number, it can be elided. The TV contains
the port number, the MO is set to "equal" and the CDA is set to "not-
sent".
If the port variation is on few bits, the TV contains the stable part
of the port number, the MO is set to "MSB" and the CDA is set to
"LSB".
If some well-known values are used, the TV can contain the list of
these values, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent".
Otherwise the port numbers are sent over the LPWAN. The TV is not
set, the MO is set to "ignore" and the CDA is set to "value-sent".
10.10. UDP length field
The UDP length can be computed from the received data. In that case,
the TV is not set, the MO is set to "ignore" and the CDA is set to
"compute-length".
If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB".
In other cases, the length SHOULD be sent and the CDA is replaced by
"value-sent".
10.11. UDP Checksum field
The UDP checksum operation is mandatory with IPv6 [RFC8200] for most
packets but recognizes that there are exceptions to that default
behavior.
For instance, protocols that use UDP as a tunnel encapsulation may
enable zero-checksum mode for a specific port (or set of ports) for
sending and/or receiving. [RFC8200] also stipulates that any node
implementing zero-checksum mode must follow the requirements
specified in "Applicability Statement for the Use of IPv6 UDP
Datagrams with Zero Checksums" [RFC6936].
6LoWPAN Header Compression [RFC6282] also authorizes to send UDP
datagram that are deprived of the checksum protection when an upper
layer guarantees the integrity of the UDP payload and pseudo-header
all the way between the compressor that elides the UDP checksum and
the decompressor that computes again it. A specific example of this
is when a Message Integrity Check (MIC) protects the compressed
message all along that path with a strength that is identical or
better to the UDP checksum.
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In a similar fashion, this specification allows a SCHC compressor to
elide the UDP checks when another layer guarantees an identical or
better integrity protection for the UDP payload and the pseudo-
header. In this case, the TV is not set, the MO is set to "ignore"
and the CDA is set to "compute-checksum".
In particular, when SCHC fragmentation is used, a fragmentation MIC
of 2 bytes or more provides equal or better protection than the UDP
checksum; in that case, if the compressor is collocated with the
fragmentation point and the decompressor is collocated with the
packet reassembly point, then compressor MAY elide the UDP checksum.
Whether and when the UDP Checksum is elided is to be specified in the
Profile.
Since the compression happens before the fragmentation, implementors
should understand the risks when dealing with unprotected data below
the transport layer and take special care when manipulating that
data.
In other cases, the checksum SHOULD be explicitly sent. The TV is
not set, the MO is set to "ignore" and the CDA is set to "value-
sent".
11. IANA Considerations
This document has no request to IANA.
12. Security considerations
12.1. Security considerations for SCHC Compression/Decompression
A malicious header compression could cause the reconstruction of a
wrong packet that does not match with the original one. Such a
corruption MAY be detected with end-to-end authentication and
integrity mechanisms. Header Compression does not add more security
problem than what is already needed in a transmission. For instance,
to avoid an attack, never re-construct a packet bigger than some
configured size (with 1500 bytes as generic default).
12.2. Security considerations for SCHC Fragmentation/Reassembly
This subsection describes potential attacks to LPWAN SCHC F/R and
suggests possible countermeasures.
A node can perform a buffer reservation attack by sending a first
SCHC Fragment to a target. Then, the receiver will reserve buffer
space for the IPv6 packet. Other incoming fragmented SCHC Packets
will be dropped while the reassembly buffer is occupied during the
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reassembly timeout. Once that timeout expires, the attacker can
repeat the same procedure, and iterate, thus creating a denial of
service attack. The (low) cost to mount this attack is linear with
the number of buffers at the target node. However, the cost for an
attacker can be increased if individual SCHC Fragments of multiple
packets can be stored in the reassembly buffer. To further increase
the attack cost, the reassembly buffer can be split into SCHC
Fragment-sized buffer slots. Once a packet is complete, it is
processed normally. If buffer overload occurs, a receiver can
discard packets based on the sender behavior, which MAY help identify
which SCHC Fragments have been sent by an attacker.
In another type of attack, the malicious node is required to have
overhearing capabilities. If an attacker can overhear a SCHC
Fragment, it can send a spoofed duplicate (e.g. with random payload)
to the destination. If the LPWAN technology does not support
suitable protection (e.g. source authentication and frame counters to
prevent replay attacks), a receiver cannot distinguish legitimate
from spoofed SCHC Fragments. Therefore, the original IPv6 packet
will be considered corrupt and will be dropped. To protect resource-
constrained nodes from this attack, it has been proposed to establish
a binding among the SCHC Fragments to be transmitted by a node, by
applying content-chaining to the different SCHC Fragments, based on
cryptographic hash functionality. The aim of this technique is to
allow a receiver to identify illegitimate SCHC Fragments.
Further attacks MAY involve sending overlapped fragments (i.e.
comprising some overlapping parts of the original IPv6 datagram).
Implementers SHOULD make sure that the correct operation is not
affected by such event.
In ACK-on-Error, a malicious node MAY force a SCHC Fragment sender to
resend a SCHC Fragment a number of times, with the aim to increase
consumption of the SCHC Fragment sender's resources. To this end,
the malicious node MAY repeatedly send a fake ACK to the SCHC
Fragment sender, with a Bitmap that reports that one or more SCHC
Fragments have been lost. In order to mitigate this possible attack,
MAX_ACK_RETRIES MAY be set to a safe value which allows to limit the
maximum damage of the attack to an acceptable extent. However, note
that a high setting for MAX_ACK_RETRIES benefits SCHC Fragment
reliability modes, therefore the trade-off needs to be carefully
considered.
13. Acknowledgements
Thanks to Carsten Bormann, Philippe Clavier, Diego Dujovne, Eduardo
Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez
Bernal, Antony Markovski, Alexander Pelov, Charles Perkins, Edgar
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Ramos, Shoichi Sakane, and Pascal Thubert for useful design
consideration and comments.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
14.2. Informative References
[RFC3385] Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna,
"Internet Protocol Small Computer System Interface (iSCSI)
Cyclic Redundancy Check (CRC)/Checksum Considerations",
RFC 3385, DOI 10.17487/RFC3385, September 2002,
.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
.
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[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, DOI 10.17487/RFC6936, April 2013,
.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, .
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
.
Appendix A. SCHC Compression Examples
This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the behavior of SCHC.
The most common case using the mechanisms defined in this document
will be a LPWAN Dev that embeds some applications running over CoAP.
In this example, three flows are considered. The first flow is for
the device management based on CoAP using Link Local IPv6 addresses
and UDP ports 123 and 124 for Dev and App, respectively. The second
flow will be a CoAP server for measurements done by the Device (using
ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to
beta::1/64. The last flow is for legacy applications using different
ports numbers, the destination IPv6 address prefix is gamma::1/64.
Figure 23 presents the protocol stack for this Device. IPv6 and UDP
are represented with dotted lines since these protocols are
compressed on the radio link.
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Management Data
+----------+---------+---------+
| CoAP | CoAP | legacy |
+----||----+---||----+---||----+
. UDP . UDP | UDP |
................................
. IPv6 . IPv6 . IPv6 .
+------------------------------+
| SCHC Header compression |
| and fragmentation |
+------------------------------+
| LPWAN L2 technologies |
+------------------------------+
DEV or NGW
Figure 23: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the Devs have a device ID.
Therefore, when such technologies are used, it is necessary to
statically define an IID for the Link Local address for the SCHC C/D.
Rule 0
+----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Comp Decomp|| Sent |
| | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+---------------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |64|1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 DevIID |64|1 |Bi| | ignore | DevIID || |
|IPv6 APPprefix |64|1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 AppIID |64|1 |Bi|::1 | equal | not-sent || |
+================+==+==+==+=========+========+============++======+
|UDP DEVport |16|1 |Bi|123 | equal | not-sent || |
|UDP APPport |16|1 |Bi|124 | equal | not-sent || |
|UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Rule 1
+----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Action || Sent |
| | | | | | Opera. | Action ||[bits]|
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+----------------+--+--+--+---------+--------+------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |64|1 |Bi|[alpha/64, match- |mapping-sent|| 1 |
| | | | |fe80::/64] mapping| || |
|IPv6 DevIID |64|1 |Bi| | ignore | DevIID || |
|IPv6 APPprefix |64|1 |Bi|[beta/64,| match- |mapping-sent|| 2 |
| | | | |alpha/64,| mapping| || |
| | | | |fe80::64]| | || |
|IPv6 AppIID |64|1 |Bi|::1000 | equal | not-sent || |
+================+==+==+==+=========+========+============++======+
|UDP DEVport |16|1 |Bi|5683 | equal | not-sent || |
|UDP APPport |16|1 |Bi|5683 | equal | not-sent || |
|UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Rule 2
+----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Action || Sent |
| | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+--------+------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |8 |1 |Up|255 | ignore | not-sent || |
|IPv6 Hop Limit |8 |1 |Dw| | ignore | value-sent || 8 |
|IPv6 DEVprefix |64|1 |Bi|alpha/64 | equal | not-sent || |
|IPv6 DevIID |64|1 |Bi| | ignore | DevIID || |
|IPv6 APPprefix |64|1 |Bi|gamma/64 | equal | not-sent || |
|IPv6 AppIID |64|1 |Bi|::1000 | equal | not-sent || |
+================+==+==+==+=========+========+============++======+
|UDP DEVport |16|1 |Bi|8720 | MSB(12)| LSB || 4 |
|UDP APPport |16|1 |Bi|8720 | MSB(12)| LSB || 4 |
|UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Figure 24: Context Rules
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All the fields described in the three Rules depicted on Figure 24 are
present in the IPv6 and UDP headers. The DevIID-DID value is found
in the L2 header.
The second and third Rules use global addresses. The way the Dev
learns the prefix is not in the scope of the document.
The third Rule compresses port numbers to 4 bits.
Appendix B. Fragmentation Examples
This section provides examples for the different fragment reliability
modes specified in this document.
Figure 25 illustrates the transmission in No-ACK mode of a SCHC
Packet that needs 11 SCHC Fragments. FCN is 1 bit wide.
Sender Receiver
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-------FCN=0-------->|
|-----FCN=1 + MIC --->| Integrity check: success
(End)
Figure 25: Transmission in No-ACK mode of a SCHC Packet carried by 11
SCHC Fragments
In the following examples, N (the size of the FCN field) is 3 bits.
Therefore, the All-1 FCN value is 7.
Figure 26 illustrates the transmission in ACK-on-Error mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
MAX_WIND_FCN=6 and no lost SCHC Fragment.
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Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4----->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2----->|
|-----W=0, FCN=1----->|
|-----W=0, FCN=0----->|
(no ACK)
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4----->|
|--W=1, FCN=7 + MIC-->| Integrity check: success
||
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->|
|-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| 6543210
||
|-----W=0, FCN=2----->|
(no ACK)
|-----W=1, FCN=6----->|
|-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->|
|- W=1, FCN=7 + MIC ->| Integrity check: failure
|| Integrity check: success
|
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Figure 28 shows an example of a transmission in ACK-on-Error mode of
a SCHC Packet fragmented in 73 tiles, with N=5, MAX_WIND_FCN=27, M=2
and 3 lost SCHC Fragments.
Sender Receiver
|-----W=0, FCN=27----->| 4 tiles sent
|-----W=0, FCN=23----->| 4 tiles sent
|-----W=0, FCN=19----->| 4 tiles sent
|-----W=0, FCN=15--X-->| 4 tiles sent (not received)
|-----W=0, FCN=11----->| 4 tiles sent
|-----W=0, FCN=7 ----->| 4 tiles sent
|-----W=0, FCN=3 ----->| 4 tiles sent
|-----W=1, FCN=27----->| 4 tiles sent
|-----W=1, FCN=23----->| 4 tiles sent
|-----W=1, FCN=19----->| 4 tiles sent
|-----W=1, FCN=15----->| 4 tiles sent
|-----W=1, FCN=11----->| 4 tiles sent
|-----W=1, FCN=7 ----->| 4 tiles sent
|-----W=1, FCN=3 --X-->| 4 tiles sent (not received)
|-----W=2, FCN=27----->| 4 tiles sent
|-----W=2, FCN=23----->| 4 tiles sent
^ |-----W=2, FCN=19----->| 1 tile sent
| |-----W=2, FCN=18----->| 1 tile sent
| |-----W=2, FCN=17----->| 1 tile sent
|-----W=2, FCN=16----->| 1 tile sent
s |-----W=2, FCN=15----->| 1 tile sent
m |-----W=2, FCN=14----->| 1 tile sent
a |-----W=2, FCN=13--X-->| 1 tile sent (not received)
l |-----W=2, FCN=12----->| 1 tile sent
l |---W=2, FCN=31 + MIC->| Integrity check: failure
e || 1 tile sent
|-----W=0, FCN=14----->| 1 tile sent
L |-----W=0, FCN=13----->| 1 tile sent
2 |-----W=0, FCN=12----->| 1 tile sent
|| 1 tile sent
T |-----W=1, FCN=2 ----->| 1 tile sent
U |-----W=1, FCN=1 ----->| 1 tile sent
|-----W=1, FCN=0 ----->| 1 tile sent
| || Integrity check: success
V |
Internet-Draft LPWAN SCHC October 2018
In this example, the L2 MTU becomes reduced just before sending the
"W=2, FCN=19" fragment, leaving space for only 1 tile in each
forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are
carried by a total of 25 SCHC Fragments, the last 9 being of smaller
size.
Note 1: Bitmaps are shown prior to compression for transmission
Note 2: other sequences of events (e.g. regarding when ACKs are sent
by the Receiver) are also allowed by this specification. Profiles
may restrict this flexibility.
Figure 29 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with
N=3, MAX_WIND_FCN=6 and no loss.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4----->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2----->|
|-----W=0, FCN=1----->|
|-----W=0, FCN=0----->|
||
|-----W=1, FCN=5----->|
|-----W=1, FCN=4----->|
|--W=1, FCN=7 + MIC-->| Integrity check: success
|
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Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->|
|-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| 6543210
||
|-----W=0, FCN=2----->|
||
|-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->|
|--W=1, FCN=7 + MIC-->| Integrity check: failure
|| Integrity check: success
||
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure
|| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2----->| Integrity check: success
|
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Figure 32 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
MAX_WIND_FCN=6, three lost SCHC Fragments, and the second SCHC ACK
lost.
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure
|| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2----->| Integrity check: success
|| ACK REQ
|
Internet-Draft LPWAN SCHC October 2018
Sender Receiver
|-----W=0, FCN=6----->|
|-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure
|| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2--X-->|
timeout| |
|--- W=0, ACK REQ --->| ACK REQ
|| Integrity check: success
|
Internet-Draft LPWAN SCHC October 2018
Sender Receiver
|-----W=0, FCN=23----->|
|-----W=0, FCN=22----->|
|-----W=0, FCN=21--X-->|
|-----W=0, FCN=20----->|
|-----W=0, FCN=19----->|
|-----W=0, FCN=18----->|
|-----W=0, FCN=17----->|
|-----W=0, FCN=16----->|
|-----W=0, FCN=15----->|
|-----W=0, FCN=14----->|
|-----W=0, FCN=13----->|
|-----W=0, FCN=12----->|
|-----W=0, FCN=11----->|
|-----W=0, FCN=10--X-->|
|-----W=0, FCN=9 ----->|
|-----W=0, FCN=8 ----->|
|-----W=0, FCN=7 ----->|
|-----W=0, FCN=6 ----->|
|-----W=0, FCN=5 ----->|
|-----W=0, FCN=4 ----->|
|-----W=0, FCN=3 ----->|
|-----W=0, FCN=2 ----->|
|-----W=0, FCN=1 ----->|
|-----W=0, FCN=0 ----->|
| |
||
|-----W=0, FCN=10----->|
||
|-----W=1, FCN=22----->|
|-----W=1, FCN=21----->|
|--W=1, FCN=31 + MIC-->| Integrity check: success
|
Internet-Draft LPWAN SCHC October 2018
+===========+
+------------+ Init |
| FCN=0 +===========+
| No Window
| No Bitmap
| +-------+
| +========+==+ | More Fragments
| | | | Send | send Fragment (FCN=0)
+===+=======+
| last fragment
| ~~~~~~~~~~~~
| FCN = 1
v send fragment+MIC
+============+
| END |
+============+
Figure 35: Sender State Machine for the No-ACK Mode
+------+ Not All-1
+==========+=+ | ~~~~~~~~~~~~~~~~~~~
| +
Internet-Draft LPWAN SCHC October 2018
+=======+
| INIT | FCN!=0 & more frags
| | ~~~~~~~~~~~~~~~~~~~~~~
+======++ +--+ send Window + frag(FCN)
W=0 | | | FCN-
Clear lcl_bm | | v set lcl_bm
FCN=max value | ++==+========+
+> | |
+---------------------> | SEND |
| +==+===+=====+
| FCN==0 & more frags | | last frag
| ~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~
| set lcl_bm | | set lcl_bm
| send wnd + frag(all-0) | | send wnd+frag(all-1)+MIC
| set Retrans_Timer | | set Retrans_Timer
| | |
|Recv_wnd == wnd & | |
|lcl_bm==recv_bm & | | +-----------------------+
|more frag | | | lcl_bm!=rcv-bm |
|~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ |
|Stop Retrans_Timer | | | Attempt++ v
|clear lcl_bm v v | +=====+=+
|window=next_window +====+===+==+===+ |Resend |
+---------------------+ | |Missing|
+----+ Wait | |Frag |
not expected wnd | | Bitmap | +=======+
~~~~~~~~~~~~~~~~ +--->+ ++Retrans_Timer Exp |
discard frag +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
| | | ^ ^ |reSend(empty)All-* |
| | | | | |Set Retrans_Timer |
| | | | +--+Attempt++ |
MIC_bit==1 & | | | +-------------------------+
Recv_window==window & | | | all missing frags sent
no more frag| | | ~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~| | | Set Retrans_Timer
Stop Retrans_Timer| | |
+=============+ | | |
| END + MAX_ACK_REQUESTS
All-1 Window & | | ~~~~~~~~~~~~~~~~~~
MIC_bit ==0 & | v Send Abort
lcl_bm==recv_bm | +=+===========+
~~~~~~~~~~~~ +>| ERROR |
Send Abort +=============+
Figure 37: Sender State Machine for the ACK-Always Mode
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Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set lcl_bm(FCN) | v v |discard
++===+===+===+=+
+---------------------+ Rcv +--->* ABORT
| +------------------+ Window |
| | +=====+==+=====+
| | All-0 & w=expect | ^ w =next & not-All
| | ~~~~~~~~~~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~
| | set lcl_bm(FCN) | |expected = next window
| | send lcl_bm | |Clear lcl_bm
| | | |
| | w=expected & not-All | |
| | ~~~~~~~~~~~~~~~~~~ | |
| | set lcl_bm(FCN)+-+ | | +--+ w=next & All-0
| | if lcl_bm full | | | | | | ~~~~~~~~~~~~~~~
| | send lcl_bm | | | | | | expected = nxt wnd
| | v | v | | | Clear lcl_bm
| |w=expected& All-1 +=+=+=+==+=++ | set lcl_bm(FCN)
| | ~~~~~~~~~~~ +->+ Wait +* ABORT
| | ~~~~~ +-------->+========+=++
| | snd lcl_bm All-1 & w=next| | All-1 & w=nxt
| | & MIC wrong| | & MIC right
| | ~~~~~~~~~~~~~~~~~| | ~~~~~~~~~~~~~~~~~~
| | set lcl_bm(FCN)| |set lcl_bm(FCN)
| | send lcl_bm| |send lcl_bm
| | | +----------------------+
| |All-1 & w=expected | |
| |& MIC wrong v +---+ w=expected & |
| |~~~~~~~~~~~~~~~~~~~~ +====+=====+ | MIC wrong |
| |set lcl_bm(FCN) | ++ +--->* ABORT |
| +===+====+=+-+ All-1&MIC wrong|
| | ^ | ~~~~~~~~~~~~~~~|
| w=expected & MIC right | +---+ send lcl_bm |
| ~~~~~~~~~~~~~~~~~~~~~~ | |
| set lcl_bm(FCN) | +-+ Not All-1 |
| send lcl_bm | | | ~~~~~~~~~ |
| | | | discard |
|All-1&w=expected & MIC right | | | |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v +----+All-1 |
|set lcl_bm(FCN) +=+=+=+=+==+ |~~~~~~~~~ |
|send lcl_bm | ++ END | |
+==========+
Internet-Draft LPWAN SCHC October 2018
--->* ABORT
~~~~~~~
Inactivity_Timer = expires
When DWL
IF Inactivity_Timer expires
Send DWL Request
Attempt++
Figure 38: Receiver State Machine for the ACK-Always Mode
+=======+
| |
| INIT |
| | FCN!=0 & more frags
+======++ ~~~~~~~~~~~~~~~~~~~~~~
Frag RuleID trigger | +--+ Send cur_W + frag(FCN);
~~~~~~~~~~~~~~~~~~~ | | | FCN--;
cur_W=0; FCN=max_value;| | | set [cur_W, cur_Bmp]
clear [cur_W, Bmp_n];| | v
clear rcv_Bmp | ++==+==========+ **BACK_TO_SEND
+->+ | cur_W==rcv_W &
**BACK_TO_SEND | SEND | [cur_W,Bmp_n]==rcv_Bmp
+-------------------------->+ | & more frags
| +----------------------->+ | ~~~~~~~~~~~~
| | ++===+=========+ cur_W++;
| | FCN==0 & more frags| |last frag clear [cur_W, Bmp_n]
| | ~~~~~~~~~~~~~~~~~~~~~~~| |~~~~~~~~~
| | set cur_Bmp; | |set [cur_W, Bmp_n];
| |send cur_W + frag(All-0);| |send cur_W + frag(All-1)+MIC;
| | set Retrans_Timer| |set Retrans_Timer
| | | | +-----------------------------------+
| |Retrans_Timer expires & | | |cur_W==rcv_W&[cur_W,Bmp_n]!=rcv_Bmp|
| |more Frags | | | ~~~~~~~~~~~~~~~~~~~ |
| |~~~~~~~~~~~~~~~~~~~~ | | | Attempts++; W=cur_W |
| |stop Retrans_Timer; | | | +--------+ rcv_W==Wn &|
| |[cur_W,Bmp_n]==cur_Bmp; v v | | v [Wn,Bmp_n]!=rcv_Bmp|
| |cur_W++ +=====+===+=+=+==+ +=+=========+ ~~~~~~~~~~~|
| +-------------------+ | | Resend | Attempts++;|
+----------------------+ Wait x ACK | | Missing | W=Wn |
+--------------------->+ | | Frags(W) +
Internet-Draft LPWAN SCHC October 2018
| No more Frags| | |
| ~~~~~~~~~~~~~~| | |
| stop Retrans_Timer;| | |
|(re)send frag(All-1)+MIC | | |
+-------------------------+ | |
cur_W==rcv_W&| |
[cur_W,Bmp_n]==rcv_Bmp&| | Attempts > MAX_ACK_REQUESTS
No more Frags & MIC flag==OK| | ~~~~~~~~~~
~~~~~~~~~~~~~~~~~~| | send Abort
+=========+stop Retrans_Timer| | +===========+
| END ++ ERROR |
+=========+ +===========+
Figure 39: Sender State Machine for the ACK-on-Error Mode
This is an example only. The specification in Section 8.4.3.1 is
open to very different sequencing of operations.
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+=======+ New frag RuleID received
| | ~~~~~~~~~~~~~
| INIT +-------+cur_W=0;clear([cur_W,Bmp_n]);
+=======+ |sync=0
|
Not All* & rcv_W==cur_W+---+ | +---+
~~~~~~~~~~~~~~~~~~~~ | | | | (E)
set[cur_W,Bmp_n(FCN)]| v v v |
++===+=+=+===+=+
+----------------------+ +--+ All-0&Full[cur_W,Bmp_n]
| ABORT *+=+=+=+=+=+====+ clear [cur_W,Bmp_n]
| | All-0 empty(Wn)| | | | ^ ^
| | ~~~~~~~~~~~~~~ +----+ | | | |rcv_W==cur_W & sync==0;
| | sendACK([Wn,Bmp_n]) | | | |& Full([cur_W,Bmp_n])
| | | | | |& All* || last_miss_frag
| | | | | |~~~~~~~~~~~~~~~~~~~~~~
| | All* & rcv_W==cur_W|(C)| |sendACK([cur_W,Bmp_n]);
| | & sync==0| | | |cur_W++; clear([cur_W,Bmp_n])
| |&no_full([cur_W,Bmp_n])| |(E)|
| | ~~~~~~~~~~~~~~~~ | | | | +========+
| | sendACK([cur_W,Bmp_n])| | | | | Error/ |
| | | | | | +----+ | Abort |
| | v v | | | | +===+====+
| | +===+=+=+=+===+=+ (D) ^
| | +--+ Wait x | | |
| | All-0 empty(Wn)+->| Missing Frags |0 & rcv_W!=cur_W & sync>0
~~~~~~~~~~~~ & Full([rcv_W,Bmp_n])
Wn=oldest[not full(W)]; ~~~~~~~~~~~~~~~~~~~~
sendACK([Wn,Bmp_n]) Wn=oldest[not full(W)];
sendACK([Wn,Bmp_n]);sync--
ABORT-->* Uplink Only &
Inact_Timer expires
(E) Not All* & rcv_W!=cur_W || Attempts > MAX_ACK_REQUESTS
~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~
sync++; cur_W=rcv_W; send Abort
set[cur_W,Bmp_n(FCN)]
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(A)(B)
| |
| | All-1 & rcv_W==cur_W & MIC!=OK All-0 empty(Wn)
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +-+ ~~~~~~~~~~
| | sendACK([cur_W,Bmp_n],MIC=0) | v sendACK([Wn,Bmp_n])
| | +===========+=++
| +--------------------->+ Wait End +-+
| +=====+=+====+=+ | All-1
| rcv_W==cur_W & MIC==OK | | ^ | & rcv_W==cur_W
| ~~~~~~~~~~~~~~~~~~~~~~ | | +---+ & MIC!=OK
| sendACK([cur_W,Bmp_n],MIC=1) | | ~~~~~~~~~~~~~~~~~~~
| | | sendACK([cur_W,Bmp_n],MIC=0);
| | | Attempts++
|All-1 & Full([cur_W,Bmp_n]) | |
|& MIC==OK & sync==0 | +-->* ABORT
|~~~~~~~~~~~~~~~~~~~ v
|sendACK([cur_W,Bmp_n],MIC=1) +=+=========+
+---------------------------->+ END |
+===========+
ABORT -->* Uplink Only &
Inact_Timer = expires
|| Attempts > MAX_ACK_REQUESTS
~~~~~~~~~~~~~~~~~~~~~
send Abort
Figure 40: Receiver State Machine for the ACK-on-Error Mode
Appendix D. SCHC Parameters
This section lists the information that need to be provided in the
LPWAN technology-specific documents.
o Most common uses cases, deployment scenarios
o Mapping of the SCHC architectural elements onto the LPWAN
architecture
o Assessment of LPWAN integrity checking
o Various potential channel conditions for the technology and the
corresponding recommended use of SCHC C/D and F/R
This section lists the parameters that need to be defined in the
Profile.
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o Rule ID numbering scheme, fixed-sized or variable-sized Rule IDs,
number of Rules, the way the Rule ID is transmitted
o Padding: size of the L2 Word (for most LPWAN technologies, this
would be a byte; for some technologies, a bit)
o Decision to use SCHC fragmentation mechanism or not. If yes:
* reliability mode(s) used, in which cases (e.g. based on link
channel condition)
* Rule ID values assigned to each mode in use
* presence and number of bits for DTag (T) for each Rule ID value
* support for interleaved packet transmission, to what extent
* WINDOW_SIZE, for modes that use windows
* number of bits for W (M) for each Rule ID value, for modes that
use windows
* number of bits for FCN (N) for each Rule ID value
* value of MAX_WIND_FCN and use of FCN values, if applicable to
the SCHC F/R mode.
* size of MIC and algorithm for its computation, for each Rule
ID, if different from the default CRC32. Byte fill-up with
zeroes or other mechanism, to be specified.
* Retransmission Timer duration for each Rule ID value, if
applicable to the SCHC F/R mode
* Inactivity Timer duration for each Rule ID value, if applicable
to the SCHC F/R mode
* MAX_ACK_REQUEST value for each Rule ID value, if applicable to
the SCHC F/R mode
o if L2 Word is wider than a bit and SCHC fragmentation is used,
value of the padding bits (0 or 1). This is needed because the
padding bits of the last fragment are included in the MIC
computation.
A Profile MAY define a delay to be added between each SCHC message
transmission to respect local regulations or other constraints
imposed by the applications.
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o Note on soliciting downlink transmissions: In some LPWAN
technologies, as part of energy-saving techniques, downlink
transmission is only possible immediately after an uplink
transmission. In order to avoid potentially high delay in the
downlink transmission of a fragmented SCHC Packet, the SCHC
Fragment receiver may want to perform an uplink transmission as
soon as possible after reception of a SCHC Fragment that is not
the last one. Such uplink transmission may be triggered by the L2
(e.g. an L2 ACK sent in response to a SCHC Fragment encapsulated
in a L2 PDU that requires an L2 ACK) or it may be triggered from
an upper layer.
o the following parameters need to be addressed in documents other
than this one but not forcely in the LPWAN technology-specific
documents:
* The way the contexts are provisioned
* The way the Rules as generated
Appendix E. Supporting multiple window sizes for fragmentation
For ACK-Always or ACK-on-Error, implementers MAY opt to support a
single window size or multiple window sizes. The latter, when
feasible, may provide performance optimizations. For example, a
large window size SHOULD be used for packets that need to be carried
by a large number of SCHC Fragments. However, when the number of
SCHC Fragments required to carry a packet is low, a smaller window
size, and thus a shorter Bitmap, MAY be sufficient to provide
feedback on all SCHC Fragments. If multiple window sizes are
supported, the Rule ID MAY be used to signal the window size in use
for a specific packet transmission.
Note that the same window size MUST be used for the transmission of
all SCHC Fragments that belong to the same SCHC Packet.
Appendix F. Downlink SCHC Fragment transmission
For downlink transmission of a fragmented SCHC Packet in ACK-Always
mode, the SCHC Fragment receiver MAY support timer-based SCHC ACK
retransmission. In this mechanism, the SCHC Fragment receiver
initializes and starts a timer (the Inactivity Timer is used) after
the transmission of a SCHC ACK, except when the SCHC ACK is sent in
response to the last SCHC Fragment of a packet (All-1 fragment). In
the latter case, the SCHC Fragment receiver does not start a timer
after transmission of the SCHC ACK.
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If, after transmission of a SCHC ACK that is not an All-1 fragment,
and before expiration of the corresponding Inactivity timer, the SCHC
Fragment receiver receives a SCHC Fragment that belongs to the
current window (e.g. a missing SCHC Fragment from the current window)
or to the next window, the Inactivity timer for the SCHC ACK is
stopped. However, if the Inactivity timer expires, the SCHC ACK is
resent and the Inactivity timer is reinitialized and restarted.
The default initial value for the Inactivity timer, as well as the
maximum number of retries for a specific SCHC ACK, denoted
MAX_ACK_RETRIES, are not defined in this document, and need to be
defined in a Profile. The initial value of the Inactivity timer is
expected to be greater than that of the Retransmission timer, in
order to make sure that a (buffered) SCHC Fragment to be
retransmitted can find an opportunity for that transmission.
When the SCHC Fragment sender transmits the All-1 fragment, it starts
its Retransmission Timer with a large timeout value (e.g. several
times that of the initial Inactivity timer). If a SCHC ACK is
received before expiration of this timer, the SCHC Fragment sender
retransmits any lost SCHC Fragments reported by the SCHC ACK, or if
the SCHC ACK confirms successful reception of all SCHC Fragments of
the last window, the transmission of the fragmented SCHC Packet is
considered complete. If the timer expires, and no SCHC ACK has been
received since the start of the timer, the SCHC Fragment sender
assumes that the All-1 fragment has been successfully received (and
possibly, the last SCHC ACK has been lost: this mechanism assumes
that the retransmission timer for the All-1 fragment is long enough
to allow several SCHC ACK retries if the All-1 fragment has not;been
received by the SCHC Fragment receiver, and it also assumes that it
is unlikely that several ACKs become all lost).
Appendix G. Note
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336, and by the ERDF and the Spanish
Government through project TEC2016-79988-P. Part of his contribution
to this work has been carried out during his stay as a visiting
scholar at the Computer Laboratory of the University of Cambridge.
Authors' Addresses
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Ana Minaburo
Acklio
1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex
France
Email: ana@ackl.io
Laurent Toutain
IMT-Atlantique
2 rue de la Chataigneraie
CS 17607
35576 Cesson-Sevigne Cedex
France
Email: Laurent.Toutain@imt-atlantique.fr
Carles Gomez
Universitat Politecnica de Catalunya
C/Esteve Terradas, 7
08860 Castelldefels
Spain
Email: carlesgo@entel.upc.edu
Dominique Barthel
Orange Labs
28 chemin du Vieux Chene
38243 Meylan
France
Email: dominique.barthel@orange.com
Juan Carlos Zuniga
SIGFOX
425 rue Jean Rostand
Labege 31670
France
Email: JuanCarlos.Zuniga@sigfox.com
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